5742 
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cop.l  — 
The  elementary 


principles  of 


Southern  Branch 
of  the 

fniversity  of  California 

Los  Angeles 


ae. 


J 


This  book  is  DUE  on  the  last  date  stamped  below 


:C  1 1  1940 
DEC  1  3  19*3 


Form  L-9-15m-8,'24 


THE  ELEMENTARY  PRINCIPLES  OF 
WIRELESS  TELEGRAPHY 


The 


Elementary  Principles 

of 

Wireless  Telegraphy 


by 

R.  D.  Bangay 


1 3 


Wireless  Press,  Inc. 

25  Elm  Street 
New  Yor 


0      7  'IS 


ALL  RIGHTS  RESERVED. 


Co 


PREFACE  TO  FIRST  EDITION 
p 

IN  presenting  this  Handbook,  the  author  has  endeavoured 
to  explain,  in  the  simplest  possible  manner,  the  theory 
and  practice  of  Wireless  Telegraphy. 

It  has  been  his  aim  to  make  the  subject  intelligible  to 
persons  who  do  not  possess  much  technical  knowledge, 
and  to  be  at  the  same  time  brief  and  accurate. 

The  book  has  been  so  arranged  as  to  be  useful  as  a 
reference  book  on  the  subject  for  students  and  amateurs 
in  this  special  branch  of  electrical  science. 

Further  and  more  complete  explanations  of  the  various 
phenomena  described  can  be  obtained  from  the  standard 
scientific  works  on  the  subject,  but  it  has  been  the  object 
of  the  author  to  deal  with  the  subject  clearly  and  simply 
without  going  too  deeply  into  the  many  highly  technical 
problems  involved. 

R.  D.  B. 


PBEFACE  TO   SECOND  EDITION 

WITH  the  object  of  increasing  the  usefulness  of  this 
Handbook,  the-  author  has  extended-its  scope  without 
going  any  more  deeply  into  the  technical  side  of  the 
subject. 

Since  the  book  has  been  used  largely  in  the  training 
of  Telegraphists  who  are  frequently  called  upon  to  take 
sole  charge  of  complete  Wireless  Telegraph  Installations, 
the  author  has  endeavoured  to  cover  all  parts  of  the 
transmitting  and  receiving  apparatus  in  such  a  way  as 
to  give  the  student  a  sound  working  knowledge  of  the 
apparatus  entrusted  to  his  care. 

For  the  convenience  6f  the  student,  the  new  edition 
is  divided  into  two  parts.  Part  I.  contains,  in  addition 
to  the  matter  published  in  the  first  edition,  and  now 
revised,  a  good  deal  of  further  information  regarding 
Receivers  and  Aerials.  In  Part  II.  the  component 
parts  of  a  Transmitter  are  explained  separately,  and 
the  theory  of  the  condition  of  resonance  under  which 
they  can  most  effectively  be  combined,  and  to  which 
each  part  should  be  adjusted  to  form  an  efficient 
transmitter,  is  fully  discussed. 

R.  D   B. 


CONTENTS 

PAOE 

MORSE  CODE       .          .          .          .  .  ix 

SYMBOLS  USED  IN  DIAGRAMS  .  .          .       xi 

ELECTRICITY  AND  MAGNETISM  ....        3 

Electro-statics — Conductors  and  insulators — Static  in- 
duction —  The  Condenser  —  Electro-dynamics — Electric 
circuits. 

Units  of  electricity — The  Coulomb — The  Ampere— The 
Volt — The  Ohm — The  Henry— The  Farad — The  Joule — 
The  Watt — Ohm's  law. 

MAGNETISM         .......       24 

Electro  -  magnetism  —  Electro  -  magnetic      induction  — 

Mutual  induction. 

The  construction  of  the  induction  coil — Production  of 

electricity  by  chemical  action — Accumulators. 

THE  PRINCIPLES  OF  WAVE  MOTION  .  .  .  .43 

Properties  of  waves— Communication  by  wave-motion 
— Measurements  of  waves. 

PRESSURE  WAVES          ...  4£ 

Aether  waves — Communication  by  means  of  aether 
waves. 

PRODUCTION  OF  WAVES  52 

Production  of  height  waves — Production  of  pressure 
waves — Production  of  electric  waves. 

PRODUCTION  OF  HIGH-FREQUENCY  OSCILLATIONS  65 

Oscillatory  circuits — Energy  and  power  in  oscillatory 
circuits — Power  in  oscillatory  circuit — Open  and  closed 
oscillatory  circuits — Variation  of  wave-lengths  of  open 
oscillatory  circuits — To  increase  the  wave-length  of  an 
aerial — To  reduce  the  wave-length  of  an  aerial— Variation 
of  wave-lengths  of  closed  oscillatory  circuits. 

PRODUCTION  OF  OSCILLATING  CURRENTS  IN  AN  AERIAL  .      84 
Direct  excitation  of  the  aerial. 


viii  WIRELESS  TELEGRAPHY 

PAGE 

COUPLED  OSCILLATORY  CIRCUITS       .  .  .  .89 

Factors  limiting  the  power  in  oscillatory  circuits — The 
auto-jigger — Reaction  of  secondary  on  primary — Re- 
sultant wave-lengths  of  coupled  circuits — Calculation  of 
the  degree  of  coupling — Methods  of  varying  the  coupling 
between  two  oscillatory  circuits. 

THE  WAVEMETER          .  .  .  .  .  .111 

The  oscillatory  circuit  of  a  wavemeter — The"  detector" 
circuit  of  a  waveraeter — The  use  of  crystals — Construction 
of  an  adjustable  condenser. 

WIRELESS  TELEGRAPH  RECEIVERS     .  .        ...     118 

Essentials  of  a  receiver — Methods  of  detecting  the 
oscillating  currents  —  The  potentiometer — Method  of 
applying  the  potentiometer  to  the"  crystal — The  two- 
circuit  receiver — Proportion  of  inductance  and  capacity 
in  secondary  oscillatory  circuit — Characteristic  curve  of 
crystal. 

The  telephone  receiver— High  resistance  telephones — 
Rectifying  properties  of  carborundum — Relation  between 
the  spark  frequency  of  the  transmitter  and  sound  pro- 
duced in  the  telephones  of  receiver — To  tune  a  receiver. 

THE  TUNING  BUZZER    .  .  .  .  .  .     146 

THE  ELECTROLYTIC  DETECTOR          .  .  .          .151 

THE  MAGNETIC  DETECTOR       .  .  .  .  .159 

"  ATMOSPHERICS  "......  165 

AERIALS 167 

Shape  of  an  aerial — Size  of  an  aerial — Height  of  an  aerial 

— The  advantage  of  using  aerials  of  a  large  capacity — The 

length  of  an  aerial. 

DISTRIBUTION  OF  POTENTIAL  AND  CURRENT  ALONG  AERIALS    177 

Distribution  of  current  in  an  aerial — Effect  on  current 
and  voltage  distribution  of  connecting  an  inductance  or 
capacity  in  series  with  an  aerial — Harmonics. 

MASTS 189 

Strain  on  masts — Buckling  of  masts — Mast  stays. 
THE  INSULATION  OF  AERIALS  .          .  .  .199 

Aerial  insulators — Earths. 

INDEX  .  .     209 


THE  EUROPEAN  OR  CONTINENTAL  MORSE  CODE 


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Punctuation  Marks 


SYMBOLS  USED  IN  DIAGRAMS  OF  WIRELESS 
TELEGRAPHY  CIRCUITS 


3. 


8. 


-I'l' 


6. 


\.  Conductor. 

2.  Conductors  crossing. 

3.  Conductors  connected. 

4.  Cell. 

5.  Battery. 

6.  Resistance  coil. 


7.  Inductive  winding. 

8.  Two    coils    having   mutual 

inductance. 

9.  Condenser. 
10.  Switch. 


SYMBOLS  USED  IN  DIAGRAMS  OF  WIRELESS 
TELEGRAPHY  CIRCUITS 


JZ. 


16. 


17. 


IB. 


—  o  o— 

—  D  0— 


^l/ 

i 


J5. 


\\.  Variable  resistance. 

12.  Variable  inductance. 

13.  Variable  condenser. 

14.  Direct  current  dynamo. 

15.  Alternating  current  dynamo. 

16.  Manipulating  key. 


17.  Spark-gap. 

18.  Aerial  wire  or  antenna. 

19.  Earth  connection. 
20    Crystal  detector. 
21.  Telephone,  receiver. 


ELEMENTARY  PRINCIPLES  OF 
WIRELESS  TELEGRAPHY 

THE  object  of  this  book  is  to  instruct  the  reader  in 
the  principles  underlying  the  construction  of  modern 
Wireless  Telegraphy  apparatus,  more  especially  as 
applied  to  small  stations. 

Although  the  same  principles  apply  equally  to 
commercial  stations  having  a  range  of  communication 
up  to  thousands  of  miles,  it  is  obvious  that  the  method 
of  applying  these  principles  will  depend  to  a  certain 
extent  upon  the  size  of  the  station.  Thus  factors  which 
require  important  consideration  when  dealing  with 
powerful  plant  will  fall  to  comparative  insignificance 
when  dealing  with  small  stations. 

Wireless  Telegraphy  is  a  special  application  of 
electrical  phenomena,  therefore  an  elementary  know- 
ledge of  the  subject  of  electricity  and  magnetism  is 
absolutely  essential  before  full  advantage  can.  be  taken 
of  a  study  of  the  principles  of  Wireless  Telegraphy. 

This  book  is  written  on  the  assumption  that  an 
elementary  knowledge  of  electricity  and  magnetism  is 
possessed  by  the  reader,  but  in  order  to  assist  the  un- 
initiated, and  for  the  purpose  of  reference,  we  have, 
in  the  first  part  of  the  book,  briefly  described  the 

1  B 


2  WIRELESS  TELEGRAPHY 

various  points  of  importance,  but  the  information  given 
•should  be  supplemented  by  a  study  of  any  of  the 
standard  text-books  on  the  subject 

Electricity  is  the  name  given  to  that  which  causes 
all  electrical  phenomena. 

Electricity  is  invisible  and  intangible,  although 
both  visible  and  tangible  effects  can  be  produced'  by  it. 
Its  exact  nature,  therefore,  can  only  be  imagined, 
but  its  effect  upon  matter  has  been  carefully  studied, 
and  from  the  study  and  classification  of  these  pheno- 
mena the  laws  governing  the  effects  of  electricity 
have  been  deduced. 

Perhaps  the  best  view  -  point  to  take  is  to 
consider  electricity  as  an  agent  or  medium  by  which 
work  or  energy  can  be  transmitted  from  one  point  to 
another. 

For  instance,  let  us  take,  as  an  example,  an  electric- 
power  installation  In  one  building  we  have  an  engine 
driving  a  dynamo,  and  in  another  building,  which  may 
be  far  removed,  we  have  an  electric  motor  driving,  say, 
a  circular  saw  used  for  cutting  wood. 

Energy  is  put  into  the  boiler  of  the  engine  m  the 
form  of  Tieat  by  burning  coal.  The  boiler  converts  this 
heat  energy  into  steam  pressure  carried  along  pipes  to 
the  engine,  where  in  turn  it  is  converted  by  the  engine 
and  dynamo  into  electrical  energy.  The  electrical 
energy  is  carried  along  wires  to  the  motor  in  the  other 
building,  and  the  motor  converts  the  electrical  energy 
into  mechanical  energy  by  turning  a  circular  saw,  and 
the  energy  is  then  used  to  cut  through  wood. 

Now  it  is  not  the  electricity  which  cuts  the  wood,  but 
the  energy  which  is  put  into  the  boiler  of  the  engine 
Electricity  is  simply  a  convenient  agent  by  which  that 


WIRELESS  TELEGRAPHY  3 

energy  can  be  transferred  from  the  boiler-room  to  the 
work-room. 

It  is  on  account  of  the  fact  that  by  this  agency 
energy  can  be  transferred  conveniently  and  cheaply 
over  a  great  or  small  distance,  and  can  by  means  of 
suitable  apparatus  be  converted  into  almost  any  desired 
form  of  energy — such,  for  example,  as  light,  heat, 
motive  power,  etc. — that  electricity  is  so  extensively 
used  for  all  purposes. 


ELECTRICITY  AND  MAGNETISM 

1.  When  a  charge  of  electricity  rests  on  the  surface 
of  any  substance,  such  as  amber,  glass,  etc.,  the  charge 
of   electricity  is   known  as  a  static  charge,  and   the 
study  ^of  the  effects  of  these  charges  is  known  as  electro- 
statics. 

2.  When  a  charge  of  electricity  passes  through  a  sub- 
stance, such  as  copper,  silver,  etc.,  the  charge  is  known 
as  an  electric  current,  and  the  study  of  the  effects  of  these 
currents  is  known  as  electro-dynamics. 

ELECTRO-STATICS 

3.  If  we  take  a  piece  of  amber  and  rub  it  with  a  piece 
of  silk,  we  find  that  the  amber  has  acquired  the  property 
of  attracting  very  light  objects,  such  as  fragments  of 
paper,  cork,  cotton-wool,  or  pith  balls,  and  that  if  these 
objects  actually  touch  the  amber  which  is  attracting 
them,  they  are  then  repelled. 

These  attractions  and  repulsions  are  due  to  a  static 
charge  of  electricity,  which  has  been  generated  by  the 


\ 


4  WIRELESS  TELEGRAPHY 

friction  with  the  silk,  and  which  is  resting  on  the  surface 
of  the  amber. 

4.  If  then,  for  convenience,  we  suspend  a  small  pith 
ball  by  a  silk  thread,  as  shown  in  Fig.  1,  and  approach 
it  with  an  electrified  amber  rod,  we  shall  see  that  the  pith 
ball  will  first  fly  towards  the  rod,  and  that  immediately 
it  touches  the  rod  it  will  be  repelled. 

5.  This  is  because  by  contact  with  the  rod  the  pith 
ball  has  itself  become  charged,  and  as  long  as  both  the 
pith  bail  and  the  amber  rod  retain  their  charges,  repulsion 

will  take  place  whenever  they  are 
brought  near  each  other. 

6.  If,  now,  instead  of  electrifying 
an  amber  rod  with  a  piece  of  silk, 
we  electrify  a  piece  of  sealing-wax 
by  rubbing  it  with  a  piece  of  flannel 
or  fur,  and  we  approach  the  already 
electrified  pith  ball  with  the  electri- 
fied sealing-wax,  we  find  that  instead 
of  repelling  the  ball  as  the  electrified 
FlQ-  *•  amber  rod  does,  it  attracts  it,  al- 

though as  soon  as  they  come  in  contact  with  each  other 
the  pith  ball  is  again  repelled,  and  after  being  repelled 
by  the  electrified  sealing-wax  will  once  more  be  attracted 
by  the  electrified  amber. 

7.  When  this  simple  experiment  is  analysed,  it  is 
found  that  there  are  two  kinds  of  electrification — one 
produced  by  rubbing  amber  with  silk,  and  the  other 
produced  by  rubbing  sealing-wax  with  fur. 

8.  In  order  to  distinguish  between  the  two,  that 
produced  by  rubbing  amber'with  silk  is  called  a  positive 
charge  ( -f ),  and  that  produced  by  rubbing  sealing-wax 
with  fur  is  called  a  negative  charge  ( - ). 


WIRELESS  TELEGRAPHY  5 

9.  By   simple   experiments   it   can   be   shown   that 
neither  charge  is  ever  produced  alone,  for  when  amber  is 
rubbed  with  silk,  although  a  positive  charge  is  produced 
on  the  amber,  an  equal  negative  charge  is  produced  at 
the  same  time  on  the  silk,  and  vice  versa  when  sealing- 
wax  is  rubbed  with  fur. 

10.  From  these  and  other  similar  experiments  the 
following  laws  may  be  deduced  : 

(1)  That  when  either  a  positive  or  a  negative  charge  is 
produced,  an  equal  and  opposite  charge  is  also  produced. 

(2)  That  like  charges  repel  one  another,  and  unlike 
charges  attract  one  another. 

(3)  That  when  an  electrified  body  touches  an  unelectrified 
body,  the  latter  becomes  charged  to  the  same  "  polarity  " 
as  the  former. 

(4)  That  when  an  electrified  body  touches  an  oppositely 
electrified  body,  if  the  two  charges  are  equal  their  electri- 
fication is  destroyed,  and  they  are  then  said  to  be  discharged ; 
but  if  the  charge  on  one  body  is  greater  than  that  on  the 
other,  their  electrification  is  only  partially  destroyed,  and 
both  bodies  become  charged  to  the  same  polarity  as  that 
of  the  greater  charge. 

CONDUCTORS  AND  INSULATORS 

11.  The  bodies  which  we  have  been  electrifying  do 
not  conduct  electricity,  but  they  resist  or  oppose  the 
passage  of  electricity  through  them,  and  it  is  for  this 
reason  that  the  charge  produced  on  them  rests  on  their 
surface. 

12.  When  a  charge  of  electricity  is  applied  to  a  metal, 
the  electricity  immediately  flows  through  it,  and  for  this 
reason  metals  are  called  conductors  of  electricity. 


6  WIRELESS  TELEGRAPHY 

13.  All  metals  are  conductors,  those  most  commonly 
used   in   electrical   apparatus   for-  this   purpose   being 
copper,  brass,  aluminium,  iron,  etc.    To  a  much  lesser 
extent  the  human  body  and  water  (except  the  purest 
distilled  water)  are  conductors. 

14.  The  substances  which  will  not  conduct  electricity 
are  called  insulators,   and  for  this  purpose  the  chief 
materials  used  in  electrical  apparatus  are  amber,  sealing- 
wax,  glass,  porcelain,  ebonite,  mica,  silk,  rubber,  oils, 
dry  wood,  string,  and  cotton. 

15.  An  important  point  to  bear  in  mind  is  that  none 
of  the  substances  mentioned  as  conductors  are  perfect 
conductors ;  that  is  to  say,  none  of  them  will  carry  a 
current  of  electricity  without  some  loss  of  energy  due  to 
"  friction  "  or  "  resistance  "    Some  of  them,  however, 
are  better  conductors  than  others ;  for  instance,  copper 
is  a  better  conductor  than  iron,  and  for  this  reason  there 
is  less  loss  of  energy  due  to  "  resistance  "  in  copper  than 
in  iron. 

16.  Similarly   no   substance   is   a   perfect   non-con- 
ductor, or  insulator.    There  will  always  be  some  loss  due 
to  leakage,  although  by  using  a  suitable  material  this 
leakage  can  be  reduced  to  a  negligible  quantity. 


STATIC  INDUCTION 

17.  When  an  electrified  conductor  is  brought  near 
another  conductor  which  has  not  been  electrified,  an 
electric  charge  will  be  induced  in  the  latter. 

This  effect  is  known  as  Static  Induction. 

18.  The  charge  which  is  induced  in  the  non-electrified 
conductor   is   not  a   permanent  charge,   but  depends 
entirely   for  its  existence  upon  its  proximity  to  the 


WIRELESS  TELEGRAPHY 


electrified  conductor  This  is  illustrated  in  Fig.  2, 
where  A  is  a  plate  df  metal  which  has  been  permanently 
charged  by  touching  it  with  an  electrified  amber  rod,  and 
B  is  a  similar  plate  of  metal  which  has  not  been  charged 
Both  plates  are  supported  by  pillars  of  glass,  or  other 
insulating  material,  to  prevent  their  charges  being  con- 
ducted to  earth. 

19.  As  A  is  brought  nearer  and  nearer  to  B  a  stronger 
and  stronger  charge  is  induced  m  the  latter,  and  as  A  is 
taken  farther  away  from  B  the  mduced  charge  in  B 
becomes  weaker.     All  the  time, 

however,  the  plate  A  retains  the 
original  charge  which  was  given 
to  it. 

20.  The  range  of  space  over 
which  the  electrified  plate  A  has 
the  power  of  inducing  a  charge 
in  B  is  called  the  Electro-static 
Field. 

21.  If    the    two    plates    are 
brought   together   so  that  they 

touch  one  another,  then  the  permanent  charge  in  A 
flows  into  B,  and  the  charge  is  divided  equally  between 
the  two  plates.  The  plate  B  will  then  retain  this  charge, 
even  when  taken  away  from  the  influence,  or  electro- 
static field,  of  A. 

22.  We  have  said  that  the  strength  of  the  charge 
mduced  by  an  electrified  conductor  in  another  con- 
ductor depends  upon  the  distance  between  the  two. 
The  strength  of  the  charge  also  depends  upon  the  nature 
of  the  substance  between  the  two  bodies,  which  must  be 
a  non-conductor. 

This  substance  is  called  a  dielectric. 


8  WIRELESS  TELEGRAPHY 

23    All  dielectrics  are*  non-conductors. 

24.  The  facility  with  which  a  dielectric  allows  static 
induction  to  act  through  it  is  called  its  Inductive 
Capacity. 

If  the  space  between  the  plates  A  and  B  is  filled 
by  glass,  it  is  found  that  a  much  stronger  charge  is 
induced  in  B  than  when  the  same  space  is  filled  with  air. 
Therefore  we  may  say  that  the  inductive  capacity  of 
glass  is  greater  than  the  inductive  capacity  of  air 

25.'  A  simple  mechanical  analogy  of  these  phenomena 

-Leather 


can  be  made  by  comparing  the  electrical  inductive 
capacity  of  a  dielectric  with  the  mechanical  extensibility 
of  a  material. 

If  we  stretch  two  pieces  of  different  material,  such  as 
a  strip  of  leather  and  a  strip  of  rubber,  each  having  an 
equal  thickness,  between  two  fixed  points,  as  shown  in 
Fig.  3,  and  we  place  on  each  of  them  a  weight  of,  say, 
1  lb.,  we  find  that  the  rubber  stretches  a  great  deal  more 
than  the  leather,  and  therefore  we  say  that  the  exten- 
sibility of  rubber  is  greater  than  that  of  leather,  just  as 
we  said  that  the  inductive  capacity  of  glass  was  p  ~ater 
than  that  of  air. 


WIRELESS  TELEGRAPHY  9 

26.  The  same  analogy  illustrates  the  effect  of  increas- 
ing the  thickness  of  the  dielectric,  for  if  we  increase  the 
thickness  of  the  rubber  strip   in   the  experiment  just 
described,  although  we  are  using  a  material  of  the  same 
extensibility  as  before,  yet  owing  to  the  fact  that  it  is 
thicker,  the  same  weight  will  not  stretch  the  rubber  to 
the  same  extent  as  before.    Similarly,  if  we  increase  the 
distance  between  the  two  plates  A  and  B  in  Fig.  2,  or, 
in  other  words,,  increase  the  thickness  of  the  dielectric, 
the  efiect  of  the  static  induction  is  reduced,  although  the 
dielectric  has  the  same  Inductive  Capacity  as  before. 

THE  CONDENSER 

27.  Static  Induction  is  the  principle  underlying  the 
construction    of    a    "  condenser."     A    simple   type    of 
condenser  consists  of  a  plate  of  glass,  or  some  other 
dielectric  (vide  paragraph  23),  covered  with  tin-foil  or 
other  conductor  on  either  side.     The  tin-foil  merely 
acts  as  a  means  of  distributing  any  applied  electrical 
pressure  uniformly  over  the  surface  of  the  dielectric. 

28.  The  property  a  condenser  has  of  holding  a  large 
or  small  charge  of  electricity  is  called  its  "  Capacity." 

This  property  of  capacity  can  best  be  illustrated  by 
comparing  it  with  the  analogous  mechanical  property 
of  springiness  or  flexibility. 

29.  If  a  mechanical  force  is  applied  to  a  spiral  spring, 
the  spring  will  be  extended  to  a  distance  X,  shown  in 
Fig.  4,  until  it  exerts  a  force  exactly  equal  and  opposite 
to  the  applied  force.    Similarly,  if  an  electrical  force 
(vide  paragraph  36)   be  applied   to   a  condenser,  the 
dielectric  of  the  condenser  will  be  strained  electrically 
until  the  condenser  exerts  an  electrical  pressure  exactly 
equal  and  opposite  to  the  force  applied  to  it 


10 


WIRELESS  TELEGRAPHY 


30  Taking  the  mechanical  case  of  a  spring,  it  will  be 
observed  that  a  movement  or  an  extension  of  the  spring 
must  take  place  before  it  exerts  an  opposite  force,  and 
further,  that  the  amount  of  this  movement.  <  t  the 
distance  X  in  Fig  4,  which  takes  place  for  a  given 
applied  force,  will  depend  upon  the  flexibility  01 
springiness  of  the  spring 

31.  Similarly,  in  the  case  of  a  condenser .  owing  to  the 
straining  of  the  dielectric,  we  get  a  certain  amount  oi 
electricity  forced  into  the  con- 
denser when  an  electrical  pres- 
sure is  applied  to  it  A  current 
of  electricity  must  flow  into  the 
condenser  before  it  exerts  an 
opposite  force,  and  further,  the 
quantity  of  electricity  which  flows 
into  a  condenser  for  a  given  ap- 
plied force  depends  upon  the 
capacity  of  that  condenser 

32  An  analogy  which  is  per- 
haps easier  to  understand  is 
shown  in  Fig  5,  where  a  steel  nozzle  is  shown,  over 
the  end  of  which  is  fastened  a  thin  india-rubber 
cap,  which  under  normal  conditions  will  lie  evenly 
across  the  end  of  the  nozzle  If  now  a  pressure  be 
applied  to  this  india-rubber  by  connecting  the  nozzle 
to  a  water-tank  the  rubber  will  be  bulged  out  by  the 
pressure  of  the  water,  as  shown  in  Fig.  6.  until  the 
rubber  exerts  a  pressure  on  the  water  equal  and  opposite 
to  the  pressure  exerted  by  the  tank 

Obviously  the  expansion  or  the  bulging  of  the  india- 
rubber  allows  it  to  contain  a  certain  quantity  of  water 
when  a  force  is  applied  to  it  so  that  we  can  say  that, 


FIG.  4 


WIRELESS  TELEGRAPHY 


11 


owing  to  the  stretching  of  the  cap,  a  certain  quantity 
of  water  will  be  forced  into  it,  and  the  quantity  of  water 
which  it  will  contain,  for  a  given  pressure,  will  depend 
upon  the  flexibility  of  the  cap. 

33.  Similarly,  if  an  electrical  force  be  applied  to  a  con- 
denser, owing  to  the  electrical  straining  of  the  dielectric 
of  the  condenser,  a  certain  quantity  of  electricity  will 
be  forced  into  it,  until  the  condenser  exerts  an  electrical 
pressure  equal  and  opposite  to  the  pressure  applied  to  it. 


Fio.  5. 


Fio.  6. 


The  quantity  of  electricity  which  will  be  forced,  into  a 
condenser  for  a  given  applied  pressure  will  depend  upon 
the  capacity  of  that  condenser. 

Thus  we  may  say  that  the  capacity  of  a  condenser 
is  analogous  to  the  flexibility  of  the  india-rubber  cap, 
or,  in  the  first  experiment  described,  to  the  flexibility 
of  the  spring ;  and  we  can  take  the  flow  of  electricity 
into  a  condenser  as  being  analogous  to  the  flow  of  water 
into  the  cap,  or,  in  the  first  experiment  described,  to 
the  movement  of  the  spring. 

34.  Now  just  as  the  flexibility  of  the  spring  (or  of 
the  cap,  as  the  case  may  be)  depends  upon  three  things, 


If  WIRELESS  TELEGRAPHY 

namely,  (1)  the  sifce  of  the  spring  (or  cap)  to  begin  with, 
i.e  its  length  and  diameter,  (2)  the  thinness  of  the 
material  of  which  it  is  made,  and  (3)  the  mechanical 
extensibility  of  that  material,  so  does  the  capacity  of  a 
condenser  depend  upon  three  analogous  factors,  namely, 
(1)  the  area  of  the  plate  forming  the  condenser,  (2)  the 
thinness  of  the  dielectric,  and  (3)  the  inductive  capacity 
of  the  dielectric. 

35  An  important  point  to  grasp  is  that  the  property 
the  condenser  has  of  holding  electricity  is  due  to  the 
electrical  straining    of   the  dielectric  and   not   to   any 
compression  of   the   electricity,  just  as   the   property 
an  india-rubber  cap  has  of  holding  water  is  due  to  the 
straining  of  the  rubber  and  not  to  any  compression  of  the 
water.    In  the  case  of  the  rubber  cap,  this  straining  has  a 
visible  effect,  inasmuch  as  it  increases  the  size  of  the  cap  , 
but  in  the  case  of  an  electrical  condenser  no  such  obvious 
result  can  be  seen,  although  electrical  instruments  will 
indicate  that  when  a  pressure  is  applied  to  the  condenser 
a  current  of  electricity  flows  into  it  until  it  is  charged. 

The  effect  of  capacity  in  an  electrical  circuit  is  in 
all  respects  similar  to  the  effect  of  a  spring  in  a  mechanical 
system,  and  the  analogy  is  extremely  helpful  in  studying 
the  effects  of  capacity  in  circuits  such  as  those  used  in 
Wireless  Telegraph  installations. 

ELECTRO-DYNAMICS 

36  An  electric  current  is  a  flow  of  electricity 

In  order  to  produce  an  electric  current,  it  is  first 
necessary  to  exert  a  difference  of  electrical  pressure 
between  two  bodies,  or  between  two  parts  of  tke  same 
body  This  difference  of  pressure  is  called  the  Eleotro- 
motive  Force. 


WIRELESS  TELEGRAPHY 


13 


37.  Ip  order  that  an  electric  current  will  flow,  how- 
ever, it  is  necessary  that  the  body,  or  bodies,  across 
which  the  difference  of  electrical  pressure  is 
exerted  is  a  conductor  of  electricity. 

A   simple  mechanical   analogy  can   be 
made  to  illustrate  this. 

A  long  pipe,  as  shown  in  Fig.  7,  is  filled 
with  water.  The  pipe  represents  a  con- 
ductor, and  the  water  illustrates  the  elec- 
tricity in  the  conductor.  Both  ends  of  the 
pipe  are  held  upwards  on  a  level  'with  one 
another,  so  that  normally  there  is  no  difference  in  pres- 
sure acting  at  each  end  of  the  tube,  and  therefore 
the  water  will  not  flow  through  the  tube. 

If,  however,  we   exert   a   pressure   at   one  end   of 
the'pipe  by  blowing  down  it,  or  by  increasing  the  height 


FIG.  7. 


FIG.  8. 


of  one  end  above  the  other,  or,  better  still,  by  connecting 
a  tank  of  water  to  it  which  is  situated  at  a  higher  level 
than  that  on  which  the  experiment  is  being  carried  out, 


14  WIRELESS  TELEGRAPHY 

as  shown  m  Fig.  8,  then  the  water  will  immediatelv  flow 
through  the  pipe. 

38  By  connecting  the  tank  to  one  end  only  of  the 
pipe,  we  exert  a  difference  of  pressure  on  the  two  ends  of 
the  pipe,  but  if  we  connect  the  tank  simultaneously  to 
both  ends  of  the  pipe,  then  there  is  no  difference  of 
pressure  on  the  two  ends  of  the  pipe,  and  consequently 
no  water  will  flow  through  it. 

39  As  the  water  represents  electricity,  the  flow  of 
water  represents  an  electric  current  (vide  paragraph  36). 

40.  The  analogy  of  water  flowing  through  a  pipe, 
although,  useful  in  some  cases,  is  not  a  good  one  to  take 
generally    as   representing  the   flow  of  electricity  in  a 
conductor,  and  it  is  apt  to  be  very  misleading,  more 
especially  when  studying  the  effects  of  capacity  and 
inductance       It  is  much  better  to  take  the  mechanical 
effect  of  "  movement "  to  represent  the  flow  of  electricity, 
and  to  take  the  nature  of  the  body  which  moves  to 
represent  the  features  of  an  electrical  circuit. 

Thus,  if  we  take  the  movement  of  a  shaft  to  represent 
the  flow  of  electricity,  we  can  take  the  bearings  in  which 
it  moves  to  represent  the  conductor  which  carries  ihe 
electric  current 

In  order  to  cause  the  shaft  to  move  in  its  bearing 
we  must  apply  a  force  or  pressure  to  the  shaft  just  as 
an  electromotive  force  must  be  applied  to  a  conductoi 
in  order  to  cause  electricity  to  flow  in  it. 

CIRCUITS 

41.  A  circuit  is  a  path  composed  of  a  conductor,  or 
conductors,  through  which   an   electric  current  flows 
from  one  point  in  it  around  the  conducting  path  back 
to  the  point  from  which  it  started. 


WIRELESS  TELEGRAPHY 


15 


An  electric  source,  such  as  a  battery,  or  dynamo, 
is  generally  included  in  a  circuit,  the  function  of  the 
electric  source  being  to  produce  a  difference  in  pressure 
or  an  electromotive  force  in  the  circuit. 


Battery. 


B 


FIG.  9. 


42.  Different  parts  of  a  circuit  can  be  connected  in 
parallel  or  in  series. 


B 


43.  Thus,  when  two  conductors  A  and  B  are  joined  in 
a  circuit  as  shown  in  Fig,  9,  they  are  said  to  be  joined  in 
parallel,  or  if  they  are  joined  as  shown  in  Fig.  10t  they 
are  said  to  be  joined  in  series. 

44.  When  conductors  are  joined  in  parallel,  only  part 


16  WIRELESS  TELEGRAPHY 

of  the  total  current  flows  through  each  conductor. 
When  they  are  joined  in  series,  the  whole  current 
passes  through  each  conductor  successively. 

45.  When  two  cells  (a  cell  is  a  source  of  electric 

pressure,  and  is  de- 
scribed later)  are  con- 
nected in  a  circuit, 
as  shown  diagram- 
matically  in  Fig.  11, 
they  are  said  to  be 
connected  in  parallel, 
and  only  part  of  the 
total  current  in  the 
Flo  n  circuit  will  flow 

through  each  cell. 

46.  When  they  are  connected,  as  shown  in  Fig.  12, 
they  are  said  to  be  connected  in  series,  and  the  whole 


FIG.  12. 

current  passing  through  the  circuit  will  flow  through 
each  cell. 

UNITS  OF  ELECTRICITY 

47.  In  order  to  measure  and  define  the  different 
electrical  factors  of  a  circuit,  certain  practical  standards, 
or  units,  have  been  adopted. 

It  is  not  necessary  for  the  purpose  of  this  book  to 
explain  how  these  units  have  been  arrived  at.  It  is 
sufficient  to  describe  the  particular  quality,  or  property, 
which  each  represents  and  the  relation  which  one  bears 
to  another. 


WIRELESS  TELEGRAPHY  17 

48.  The  unit  of  quantity  is  one  Coulomb. 

„      „      current  „  one  Ampere. 
",,      ,,     electromotive  force 

or  pressure  „  one  Volt. 

,,       ,,      resistance  ,,  one  Ohm. 

,,      „     inductance  „  one  Henry. 

„      „     capacity  „  one  Farad. 

„      „     energy  ,,  one  Joule. 

„      „     power  „  one  Watt. 

THE  COULOMB 

50.  The  Coulomb  is  the  electrical  unit  of  quantity, 
and  can  be  compared  with  the  water  unit  of  quantity, 
namely,  "  a  gallon,"  or,  better,  with  the  mechanical  unit 
of  rotary  motion,  namely,  "  one  revolution  "  (vide  para- 
graph 40). 

THE  AMPERE 

51.  The  Ampere  is  the  electrical  unit  of  current. 
When  a  current  of  water  flows  through  a  pipe,  the 

amount  of  flow  can  be  defined  by  stating  how  many 
"  gallons  per  second  "  are  flowing. 

Similarly  in  an  electrical  circuit,  when  a  current  of 
electricity  flows  through  a  conductor,  the  rate  of  flow 
can  be  defined  by  stating  how  many  coulombs  per 
second  are  flowing. 

Again,  when  a  shaft  rotates  in  a  bearing  the  amount 
of  rotation  or  "  speed  "  can  be  defined  by  stating  how 
many  revolutions  per  second  it  is  making,  and  in  this 
case  one  revolution  per  second  is  the  unit  of  speed. 

52.  In  an  electrical  circuit  one  "  ampere  "  represents 
a  flow  of  one  coulomb  of  electricity  per  second. 

THE  VOLT 

53.  The    Volt   -is    the    unit   of    electrical    pressure, 


18  WIRELESS  TELEGRAPHY 

variously  described   as    "  difference    of   potential,"  or 
"  electromotive  force  "  (E.M.F.). 

54.  It  can  be  compared  with  the'  practical  unit  of 
mechanical  force,  namely,  the  pound. 

The  flow  of  water,  that  is,  the  number  of  gallons  per 
hour  that  will  flow  through  a  pipe  of  given  length,  size, 
and  shape,  will  depend  upon  the  number  of  pounds  of 
pressure  applied  at  one  end  of  the  pipe,  or  to  put  it 
more  correctly,  upon  the  difference  in  the  number  of 
pounds  acting  on  the  two  ends  of  the  pipe. 

Again,  the  speed  at  which  a  shaft  will  rotate  in  a 
given  bearing  will  depend  upon  the  twisting  force  or 
"  torque  "  applied  to  the  shaft. 

55.  Similarly,  the  now  of  electricity,  or  the  number 
of  amperes  that  will  flow  through  a  conductor  of  given 
length,  size,  and  shape,  will  depend  upon  the  difference  in 
the  number  of  volts  acting  at  each  end  of  the  conductor. 

THE  OHM 

56.  The  Ohm  is  the  unit  of  resistance. 

57.  Resistance  can  be  compared  with  the  mechanical 
property   of   friction,   for   example,   with   the   friction 
between  the  water  and  the  inside  of  a  pipe  when  the 
water  is  flowing  through  the  pipe,  or  the  friction  between 
a  shaft  and  its  bearings. 

58.  Just  as  friction  opposes  the  flow  of  water  through 
a  pipe  or  the  rotation  of  the  shaft,  so  does  resistance 
oppose  the  flow  of  electricity  through  a  conductor. 

59.  A  conductor  having  a  resistance  of  one  ohm  unll 
require    an   electromotive  force    of  one  volt  to  force    a 
current  of  one  ampere  through  it. 

THE  HENRY 

60.  The  Henry  is  the  unit  of  inductance. 


WIRELESS  TELEGRAPHY  19 

61.  Inductance  is  that  quality  in  a  circuit  which  tends 
to  oppose  any  change  in  the  flow  of  electricity.    It  must 
not  be  confused  with  "  resistance,"  which  opposes  the 
flow  of  electricity. 

62.  It  can  best  be  described  by  comparison  with  the 
mechanical  property  of  mass,  as  its  effect  in  an  electrical 
circuit  is   analogous  to  the  effect  of  the  inertia  and 
momentum  of  a  heavy  body  in  motion. 

All  bodies  when  stationary  show  a  tendency  to  oppose 
being  put  in  motion,  or  if  they  are  already  in  motion, 
to  oppose  being  accelerated.  This  tendency  is  called 
"  inertia."  Similarly,  all  bodies  when  in  motion  show 
a  tendency  to  oppose  being  stopped,  or  having  their 
speed  reduced.  This  tendency  is  called  "  momentum." 

63.  It  is  well  known  that  it  takes  a  considerable 
time  for  an  engine  with  a  heavy  fly-wheel  to  get  up  full 
speed.     This  is  due  to  the  inertia  of  the  fly-wheel.    Also 
an  engine  running  at  full  speed  takes  a  considerable 
time  to  be  brought  to  a  standstill.    This  is  due  to  the 
momentum  of  the  fly-wheel. 

64.  In  the  same  way  there  is  a  tendency  in  a  circuit 
to  oppose  any  increase,   or  decrease,   in  the  current 
flowing  through  it.     This  quality  is  called  Inductance. 

If  an  E  M.F.  be  applied  to  a  circuit  possessing  induct- 
ance, the  current  flowing  as  a  result  of  the  E.M.F.  will 
only  gradually  grow,  and  the  greater  the  inductance,  the 
slower  the  rate  of  growth.  Again,  if,  when  the  current 
is  flowing  through  a  circuit  possessing  inductance,  the 
E.M.P.  which  is  making  it  flow  be  suddenly  removed, 
the  current  will  only  gradually  stop  flowing,  unless  of 
course  the  circuit  be  "  broken  "  or  interrupted. 

Tli  us  it  will  be  seen  that  the  effect  of  inductance  in 
a  circriit  on  any  current  flowing  through'  it  is  exactly 


20  WIRELESS  TELEGRAPHY 

similar  to   the   effect   of  inertia    in    a   body   on   any 
movement  of  that  body. 

65.  Inductance  is  really  due  to  the  magnetic  field 
produced  by  the  current  in  a  circuit  (see  paragraph  85), 
and  the  amount  of  the  inductance  depends  upon  the 
strength  of  the  magnetic  field  thus  produced,  just  as  the 
amount  of  the  inertia,  or  momentum,  of  a  fly-wheel 
depends  upon  the  weight  of  that  fly-wheel. 

66.  It  will  be  shown  later  that  the  amount  of  magnetic 
field  produced  by  a  circuit,  and  therefore  the  inductance 
of  a  circuit,  depends  upon  its  form  ;  for  instance,  the  in- 
ductance of  a  given  length  of  wire  will  be  far  greater 
if  that  wire  is  wound  into  a  coil  than  if  it  is  stretched 
o^it  straight. 

Similarly,  the  momentum  of  a  fly-wheel  depends  upon 
its  shape  as  well  as  its  weight .  thus  a  fly-wheel  two  feet 
in  (pameter,  weighing  ten  pounds,  will  have  a  very  much 
greater  momentum  or  inertia  than  a  fly-wheel  one  foot 
in  diameter,  also  weighing  ten  pounds 

67.  One  great  difference  between  the  effect  of  resist- 
ance and  that  of  inductance  in  a  circuit  is  that  resistance 
absorbs  energy  and  dissipates  it  in  the  form  of  heat, 
just  as  friction  absorbs  mechanical  energy  and  dissipate? 
it  in  the  form  of  heat,  whereas  inductance  only  stores  up 
energy  when  the  current  is  increasing,  and  gives  its  energj 
back  when  the  current  is  decreasing,  just  as  a  fly- wheel 
stores  up  energy  when  its  speed  is  increased  and  gives 
back  its  energy  when  the  speed  is  decreased. 

When  a  circuit  has  an  inductance  of  one  henry,  the 
current  flowing  through  that  circuit  will  change  by  one 
ampere  when -a  difference^/  potential  of  one  volt  has  been 
applied  for  one  second 

The  microhenrv  is  sometimes  used  as  a  more  con- 


WIRELESS  TELEGRAPHY  21 

venient  unit  when  the  circuits  under  consideration  have 
very  small  inductances.  One  microhenry  is  one- 
millionth  part  of  a  henry. 

THE  FARAD 

68.  The  Farad  is  the  unit  of  capacity. 

69.  We  have  already  described  in  paragraph  28  that 
capacity  is  the  property  which  a  condenser  has  of  holding 
a  cercain  quantity  of  electricity,  and  we  compared  this 
property  with  the  mechanical  flexibility  of  a  spring 

If  we  take  the  amount  in  inches  that  a  spring  will 
extend  when  a  given  force  of  one  pound  is  applied  to  it 
as  being  a  measure  of  its  flexibility,  we  could  specify 
this  quality  of  any  spring  by  stating  how  many  inches 
or  what  fraction  of  an  inch  theoretically  it  would  expand 
for  this  given  pressure.  Thus  by  coining  a  word, "pound- 
inch,"  we  could  say  that  if  a  spring  was  such  that  it 
would  expand  one  inch  when  one  pound  of  pressure  was 
applied  to  it,  it  has  a  flexibility  of  one  "  pound-inch." 

70.  Similarly,  the  electrical  unit  of  capacity  is  a 
measure  of  the  quantity  of  electricity  which  will  flow  into 
a  condenser  when  a  pressure  of  one  volt  is  applied  to  it. 
Thus  if  a  condenser  be  of  such  dimensions  that  it  will  hold 
one  coulomb  of  electricity  when  a  pressure  of  one  volt  is 
applied  across  it,  it  will  have  a  capacity  of  one  "  Farad." 

71.  A  condenser  sufficiently  large  to  hold  a  charge  of 
one  coulomb  of   electricity  at   a  pressure  of  one  volt 
would  have  to  be  of  enormous  dimensions,  and  therefore 
a  farad  is  too  large  a  unit  for  practical  convenience,  and 
the  microfarad  is  therefore  usually  adopted  in  its  plaro, 
a  microfarad  being  one-millionth  part  of  a  farad 

72    An  important  point  to  grasp   is  that  although 
energy  is  expended  in  charging  up   a   condenser,  this 


22  WIRELESS  TELEGRAPHY 

energy  is  in  reality  only  stored  up  by  the  condenser,  and 
is  available  for  use  by  discharging  that  condenser  through 
a  useful  channel;  just  as  in  the  case  of  a  spring  although 
energy  is  expended  in  expanding  or  compressing  a 
spring,  that  energy  is  only  stored  up  by  the  spring,  and 
is  available  for  use  by  discharging  the  spring  in  a  useful 
way.  For  example,  take  the  case  of  an  air-gun  energy 
is  stored  in  the  gun  by  compressing  a  spring,  and  this 
energy  is  then  available  for  driving  a  shot  against  the 
friction  of  fehe  air  when  the  spring  is  released 

Later  on  in  this  book  we  shall  show  many  examples 
of  how  energy  stored  up  in  a  condenser  is  made  to  do 
useful  work  by  discharging  that  condenser  through 
suitable  circuits. 

THE  JOULE 

73V  The  Joule  is  the  practical  unit  of  electrical 
energy  or  work 

la  order  to  cause  a  current  of  electricity  to  flow  in  a 
circuit,  energy  or  work  must  be  expended. 

The  same  rule  applies  to  all  matter  For  example, 
m  order  to  cause  a  body  to  move,  energy  or  work  must 
be  expended  The  mechanical  unit  of  work  is  the 
"  foot-pound,"  and  can  be  defined  as  follows 

If  a  force  of  one  pound  is  used  to  move  a  body,  the 
amount  of  work  expended  in  moving  that  body  a  distance 
of  one  foot  is  one  foot-pound.  For  instance  one  foot- 
pound of  energy  is  expended  in  lifting  a  body  weighing 
one  pound  a  foot  off  the  ground,  because  the  force  of  one 
pound  is  being  exerted  on  it  throughout  the  'distance  it 
is  being  lifted 

The  electrical  unit  of  work  is  as  we  have  already 
stated,  the  joule,  and  can  be  denned  as  follows  . 


WIRELESS  TELEGRAPHY  23 

74.  //  a  force  of  one  volt  is  used  to  cause  an  electric 
current  to  flow  through  a  circuit,  one  joule  of  work  has  been 
expended  when  one  coulomb  of  electricity  has  flowed. 

From  this  definition  of  a  joule  in  terms  of  quantity 
and  pressure,  it  follows  from  the  fact  that  one  ampere  of 
current  is  one  coulomb  per  second,  that  a  joule  is  also 
the  amount  of  energy  expended  during  one  second  of 
time  in  causing  one  ampere  to  flow  through  a  resistance 
of  one  ohm  (vide  paragraph  59). 

THE  WATT 

75.  The  Watt  is  the  electrical  unit  of  power.     Power 
is  the  work  done  per  unit  time,  or  the  rate  of  doing 
work.     One  watt  is  the  power  required  to  do  one  joule 
of  work  per  second. 

76.  Now,  since  a  flow  of  one  coulomb  per  second  is 
one  ampere,  it  follows  from  the  definition  of  a  joule  that 
one  watt  of  power  is  expended  when  one  volt  is  used 
to  cause  a  current  of  one  ampere  to  flow.     This  can  be 
expressed  as  an  equation  : 

Watts  =  Volts  x  Amperes. 

For  convenience  the  Kilo-watt  (KW)  is  often  used 
as  the  unit  of  electrical  power  instead  of  the  watt ; .  one 
kilo-watt  equals  1000  watts. 

Example. — If  a  force  of  10  volts  causes  a  current  oi 
100  amperes  to  flow  through  a  given  circuit,  the  power 
expended  in  this  circuit : 

-10  x  100  -1000  Watts  or  1  KW. 

OHM'S  LAW 

77.  In  every  electrical  circuit  there  are  particularly 
three  factors,  the  true  relation  of  which  must  be  clearly 
understood. 


24  WIRELESS  TELEGRAPHY 

These  three  factors  are  the  force  or  pressure,  the  cur- 
rent, and  the  resistance,  and,  as  already  explained,  are 
measured  in  terms  of  volts,  amperes,  and  ohms,  re- 
spectively. 

They  bear  a  definite  relation  to  one  another,  which 
js  expressed  by  Ohm's  Law. 

78.  Ohm's  Law. — The  strength  of  the  current  flawing 
through  any  circuit  is  directly  'proportional  to  the  pressure 
acting  across  the  circuit  and  inversely  proportional  to  the 
resistance  of  the  circuit. 

In  other  words,  the  current  is  equal  to  the  pressure 
divided  by  the  resistance. 

Using  the  units  which  are  a  measure  of  these  factors, 
the  law  can  be  stated  as  an  equation  thus : 

Amperes- ™s 
Ohms 

and  therefore  by  transposing, 

Volts  =  Amperes  x  Ohms 

Volts 
Ohms  =  — 

Amperes. 

Example. — If  a  pressure  of  6  volts  be  applied  to  &, 
circuit  whose   resistance  is  3  ohms,  then   the  current 
flowing  through  that  circuit  will  be  2  amperes,  thus  : 
Amperes  =  §  =  2. 

MAGNETISM 

79.  Magnetism  is  the  name  given*  to  the  power  which 
a  magnet  has  of  attracting  iron  or  other  magnetic  sub- 
stances. 

The  lodestone  is  a  natural  magnet,  and  if  a  piece 
of  hard  steel  is  rubbed  by  it,  or  by  another  magnet,  it  will 


WIRELESS  TELEGRAPHY  25 

be  found  to  act  in  the  same  way  as  the  natural  magnet 
itself;  that  is  to  say,  it  will  point  north  and  south 
when  freely  suspended,  and  will  attract  iron  filings. 

The  piece  of  steel  is  then  said  to  be  magnetised,  and 
is  known  as  a  permanent  magnet.  One  end  of  the  magnet 
is  called  the  North  (N.)  Pole,  and  the  other  the  South 
(S.)  Pole. 

80.  The  range,  or  space,  over  which  a  magnet  will 
attract  other  magnetic  substances  is  called  the  "  mag- 
netic field." 

81.  If  the  North  Pole  of  one  magnet  is  brought  near 
the  South  Pole  of  another  magnet,  the  two  will  attract 
one  another,  but  if  the  two  North  Poles  or  two  South 
Poles  are  brought  near  one  another,  they  repel  each  other. 

It  can  be  said,  therefore,  that  like  'poles  repel  and 
unlike  poles  attract  one  another. 

82.  Magnetic  effects  act  in  a  definite  direction  along 
imaginary  lines,  called  "  lines  of  force." 

Every  line  of  force  passes  out  from  the  North 
Pole  round  a  com- 

plete circuit,  and  re-    /      '  /''"  \ 

turns  into  the  South    *         ',*•-"  ~--N    \         .' 

Pole,    as    shown    in      \v    \    t     ,*---*--->     \    /    y' 


83.  When  a  mag-       <  -  '      -   I""""""  "  ~~s    v  "  -  * 
netic     substance     is    /      ,    (  /  ^      \ 

brought  into  a  mag-   i          \        ~  --_.«__-'       / 
netic   field,  the   sub-    \  /' 

stance  becomes  mag-  F       g 

netised. 

This  effect  is  called  "  magnetic  induction." 
For  the  purpose  of   this  book  it  is  not  necessary 
to  go  fully  into  the  factors  controlling  magnetic  force, 


26  WIRELESS  TELEGRAPHY 

or  the  units  by  which  these  factors  are  measured,  but 
it  is  essential  that  the  relation,  or  connection,  between 
electricity  and  magnetism  is  thoroughly  understood. 

84.  The  study  of  the  relation  between  electricity  and 
magnetism  is  called  "  electro-magnetism." 

E  LECTRO-  M  AGNETIS  M 

85.  A  current  of  electricity  passing  through  a  con- 
ductor produces  a  magnetic  field  round  that  conductor, 

/' \^~^/^"'\^"^^ 

r/;>N/VN/vx/:x> 

i  ,  ,  \/\  /  r\i\*  .•"»  /x  '\/-N  * 


FIG.  14 

the  Hues  of  force  forming  the  magnetic  field  being  a 
number  of  concentric  circles  with  the  conductor  as  their 
centre. 

86.  If  the  lines  of  force  were  visible,  a  side  view  of 
the  conductor  would  appear  as  shown  in  Fig.  14,  and  an 
end  view  as  shown  in  Fig.  15. 

87.  These  lines  of  force  have  a  definite  direction  de- 

pending upon  the  direction  in  which 
'    - —  N^  \      the  current  is  flowing  (vide  paragraph 
/  '  '^\  x  v    82). 

,'  /  /        88.  In  Fig.  15  the  direction  of  the 
<' /     lines  is  shown,  assuming  that  the  cur- 
rent is  flowing  in  the  conductor  upwards 
towards    the    reader.     If   the   current 
were    reversed,   the    direction    of    the 
lines  of  force  would  be  reversed,  although  they  would 
still  remain  as  concentric  circles. 


WIRELESS  TELEGRAPHY 


27 


FIG.  16. 


89.  If  the  conductor  is  bent  into  a  circle,  as  shown  in 
Fig.  16,  and  the  current  is  passed  through  it  in  the 
direction    shown    by 

the  large  arrows,  it 
will  be  seen  that  the 
magnetic  lines  are  all 
acting  in  an  upward 
direction  on  the  inside 
of  the  circle  of  wire, 
and  in  a  downward1 
direction  on  the  out- 
side of  the  circle.  The 
field  thus  produced  is 
exactly  similar  to  that  produced  by  a  magnet,  for  it- 
has  polarity, 

90.  Since  the  lines  of  force  come  out  of  the  upper  side 

_  _  of   the  circle  and  go 

in  at  the  under  side  of 
the  circle,  the  upper 
side  becomes  the 
North  Pole  and  the 
lower  side  becomes 
the  South  Pole. 

91.  This  effect  is 
still  more  marked  if, 
instead  of  making 
only  one  turn  of  the 
wire,  we  make  a  coil 
of  wire,  as  shown  in 
.Fig-  17. 

92.  In  this  case  the  lines  of  force  produced  by  each 

arn,  instead  of  acting  right  round  the  conductor,  can 

be  imagined  to  combine  with  those  produced  by  the 


FIG.  17. 


28  WIRELESS  TELEGRAPHY 

next  turn,  thus  giving  the  resultant  effect  shown  in 
Fig.  17. 

93.  It  may  be  said,  then,  that  if  a  straight  coil  is 
made  by  wrapping  wire  round  a  bobbin,  and  a  current 
of  electricity  from  a  battery  is  passed  through  the  coil, 
it  will  be  found  that  the  coil  behaves  exactly  as  if  it  were 
a  magnet. 

94.  If  we  insert  a  rod  of  hard  steel  into  the  coil  and 
pass  the  current  as  before,  the  steel  rod  will  become  a 
permanent  magnet. 

95.  If  instead  of  the  steel  we  insert  a  rod  at  solt  iron 
into  the  coil,  it  also  becomes  a  magnet,  but  it  is  only 
magnetic  so  long  as  the  current  lasts.     This  is  called 
an  electro-magnet. 

96.  It  is  found  that  the  strength  of  the  magnetic 
field  produced  depends  upon  three  factors  :     (1)   the 
current    passing    round    the    coil,    (2)   the   number   of 
turns  in  the  coil,  and  (3)  the  "  reluctance  "  (which  is 
the  magnetic  equivalent  of  electrical  resistance)  of  the 
"magnetic  path"  or  "magnetic  circuit." 

The  first  two  of  these  factors  taken  together  constitute 
the  force-producing  magnetism,  which  is  called  Magneto- 
motive Force. 

The  unit  of  magnetomotive  force  is  one  ampere- turn. 

97.  If  we  place  any  magnetic  substance,  such  as  iron, 
in  the  path  of  the  magnetic  lines  of  force,  the  reluctance 
of  the  path  of  the  lines  of  force  is  very  much  reduced 
because  the   "  permeability "    (which  is   the   magnetic 
equivalent  of  electrical  "  conductivity  ")  of  iron  is  very 
much  greater  than  that  of  air,  with  the  result  that  the 
strength  of  the  magnetic  field,  or,  in  other  words,  the  total 
number  of  magnetic  lines  of  force,  produced  by  the  same 
current  passing  through  the  coil,  is  very  much  increased 


WIRELESS  TELEGRAPHY 


29 


ELECTRO-MAGNETIC  INDUCTION 

98.  As  a  magnet  is  made  to  enter  a  coil  of  wire,  an 
electromotive  force  is  induced  in  the  coil  of  wire,  so  that 
if  the  electrical  circuit  be  completed  by  connecting  the 
two  ends  of  the  coil  together,  a  current  of  electricity 
will  flow  through 
the  coil. 

This  effect 
is  known  as 
Electro-magnetic 
Induction. 

99.  If  a  galva- 
nometer or  other 
suitable  measur- 
ing instrument 
be  -  connected 
between  the  ends 
of  the  coil,  as 
shown  in  Fig.  18, 
so  that  any  current  flowing  through  the  coil  will  flow 
through  the  instrument,  the  deflection  of  the  pointer  will 
indicate  roughly  the  amount  of  current  flowing  through 
the  coil,  and  the  direction  in  which  it  is  flowing. 

100.  If  now  a  magnet  be  thrust  into  the  coil,  the  needle 
of  the  galvanometer  will  be  deflected  from  its  normal 
position,  indicating  that  a  current  of  electricity  has  been 
generated  in  the  coil. 

101.  If  the  magnet  be  left  lying  inside  the  coil,  the 
needle  of  the  galvanometer  will  return  to  its  normal 
position,  thus  indicating  that  the  current  in  the  coil 
has  ceased. 

102.  We  may  say,  then,  that  a  current  of  electricity 


FIG.  18. 


30  WIRELESS  TELEGRAPHY* 

will  be  induced  in  a  coil  of  wire  by  a  magnet  so  long  as 
there  is  a  relative  movement  between  the  coil  and  the 
magnetic  field,  or,  in  other  words,  when  there  is  a  change 
in  the  number  of  lines  of  force  passing  through  the  coil. 

103.  If  we  continue  the  experiment  and  withdraw  the 
magnet  from  the  coil,  the  needle  of  the  galvanometer 
will  again  be  deflected,  but  this  time  in  the  opposite 
direction,  indicating  that  a  current  of  electricity  has  been 
generated  in  the  coil  in  the  opposite  direction  to  that 
produced  by  thrusting  the  magnet  into  the  coil. 

In  effect,  thrusting  a  magnet  into  a  coil  is  equivalent 
to  increasing  the  number  of  lines  of  force  passing  through 
the  coil,  and  vice  versa,  withdrawing  the  magnet  from  the 
coil  is  equivalent  to  decreasing  the  number  of  lines 
passing  through  the  coil. 

104.  We  may  say,  then,  that  the  direction  of  the 
current  induced  in  a  coil  by  a  relative  movement  between 
it  and  a  magnetic  field  depends  upon  whether  the  move- 
ment tends  to  increase  or  to  decrease  the  magnetic  lines 
of  force  passing  through  the  coil. 

105.  By  similar  experiments  it  will  be  found  that 
the  quicker  we  thrust  the  magnet  into  the  coil,  the  greater 
will  be  the  current  induced  in  the  coil ;  also  that  a  stronger 
magnet,  that  is  to  say,  a  magnet  with  a  greater  number 
of  lines  of  force,  will  induce  a  greater  current  in  the  coil 
than  a  weak  magnet,  even  though  the  two  be  thrust 
into,  or  withdrawn  from,  the  coil  at  the  same  speed. 

106.  We  may  say,  then,  that  the  current  induced 
in  a  coil  depends  upon  the  rate  of  change  in  the  number 
of  magnetic  lines  of  force  passing  through  the  coil. 

In  the  above  explanations  we  have  taken  the  point 
of  view  that  currents  were  generated  in  the  coil.  It 
must  be  remembered,  however,  that  this  is  not,  strictly 


WIRELESS  TELEGRAPHY 


31 


speaking,  accurate.  It  is  really  an  electromotive  force 
that  is  induced  in  the  coil,  and  the  current  only  flows 
as  a  result  of  this  electromotive  force  when  the  circuit 
through  the  coil  is  completed. 

The  word  current  is  used  merely  to  avoid  complica- 
tions. 

107.  Another  variation  can  be  made  in  these  experi- 
ments which  greatly  affects  the  amount  of  current 


FIG.  19 

induced  in  a  coil,  namely,  the  number  of  turns  of  wire 
of  which  the  coil  is  composed. 

108.  If  we  wind  two  separate  coils,  one  with  say  100 
turns  of  wire  and  the  other  with  200  turns  of  wire,  we 
find  that  twice  as  much  current  is  generated  when  we 
thrust  a  magnet  into  the  larger  coil  as  when  we  thrust 
the  same  magnet  at  the  same  speed  into  the  smaller  coil. 

The  best  way  to  try  this  experiment  is  to  connect 
both  coils  in  series  with  the  galvanometer,  as  shown  in 
Fig.  19,  and  to  introduce  the  magnet  into  one  coil  at  a 
time. 


32  WIRELESS  TELEGRAPHY 

By  arranging  it  this  way  the  resistance  of  the  circuit 
remains  the  same,  whichever  coil  is  used. 

109.  We  may  say,  then,  that  the  electromotive  force 
induced  in  a  coil  is  proportional  to  the  rate  of  change  of 
magnetic  lines  passing  through  the  coil,  and  also  to  the 
number  of  turns  of  wire  in  that  coil,  or,  to  put  it  as  an 
equation : 

110.  Electromotive   force=rate   of   change   of   lines 
x  number  of  turns. 

MUTUAL  INDUCTION 

111.  The  effects  we  have  been  considering  up  to  the 
present    are    those    produced    by    "  Electro-magnetic 
Induction." 


'20. 


Referring  to  paragraph  89,  \vc  showed  how  a  coil  of 
wire  through  which  an  electric  current  was  passirig 
produced  a  magnetic  field  similar  to  that  produced  by 
a  permanent  magnet. 


WIRELESS  TELEGRAPHY 


33 


112.  It  is  obvious,   then,   that  in  the  experiments 
described  in  the  last  paragraphs  we  can  produce  exactly 
the  same  results  by  replacing  the  magnet  by  a  coil  of 
wire  through  which  a  current  is  kept  flowing. 

113.  The  effects  then  produced  are  known  as  those  of 
Mutual  Induction,  and  an  illustration  of  this  is  shown  in 
Fig.  20. 

114.  In  the  case  of  Mutual  Induction,  though,  it  is 
not  necessary    to 

move  the  first  coil 
in  and  out  of  the 
second  coil,  for 
we  can  produce 
exactly  the  same 
effect,  namely, 
that  of  changing 
the  number  of 
lines  of  force  pass- 
ing through  the 
second  coil,  by 
leaving  the  first  Flo  .,, 

coil     permanently 

inside  the  second  coil,  and  making  and  breaking  the 
battery  circuit  through  the  first  coil  by  means  of  a 
switch,  as  shown  in  Fig  21 

115  In  these  cases  the  coil  P  through  which  tbo  cur- 
rent is  flowing  :s  called  the  Primary  Coil,  and  the  coil 
S,  in  which  the  current  is  induced,  is  called  the  Secondary 
Coil. 

110  By  relerring  to  paragraph  (J7,  it  is  obvious  that 
the  voltage  induced  in  the  secondary  coil  will  be  greatly 
increased  in  the  above  experiments  if  a  core  of  iron  is 
placed  through  the  primary  coil  P. 

D, 


34  WIRELESS  TELEGRAPHY 

117.  If  an  iron  core  is  used,  the  iron  should  be  soft, 
for  the  following  reason. 

Soft  iron  will  retain  only  a  very  small  amount  of 
the  magnetism  induced  in  it  after  the  current  passing 
round  it  has  been  interrupted.  Hard  iron,  or  steel, 
on  the  other  hand,  retains  a  very  large  portion  of  its 
magnetism  after  the  current  passing  round  it  has  ceased 
to  flow. 

The  result,  therefore,  of  using  a  steel  core  would  l)e 
that  only  a  small  change  in  the  total  number  of  magnetic 
lines  passing  through  the  secondary  would  be  obtained 
by  making  and  breaking  the  battery  circuit. 

THE  CONSTRUCTION  OF  THE  INDUCTION  COIL 

118.  An  Induction  Coil  is  an  instrument  for  producing 
high-voltage  impulses. 

It  is  constructed  on  the  principles  of  electro-magnetic 
induction,  which  we  briefly  described  in  paragraphs  98 
to  117. 

119.  In  paragraph  114  we  showed  how  a  current  of 
electricity  could  be  produced  in  a  secondary  coil  by 
making  and  breaking  the  battery  circuit  through  a 
primary  coil. 

In  paragraph  109  we  showed  how  the  voltage,  or 
pressure,  of  the  electricity  induced  in  the  secondary 
coil  depends  upon  two  things  : 

(1)  The  rate  of  change  in  the  number  of  magnetic 
lines  of  force  which  pass  through  the  secondary  coil. 

(2)  The  number  of  turns  of  wire  with  which  the 
secondary  coil  is  wound. 

120.  The  quicker  the  rate  of  change  in  the  number 
of  magnetic  lines  of  force  the  greater  the  resultant 


WIRELESS  TELEGRAPHY  35 

voltage  across  the  secondary  coil.  Also,  the  greater  the 
number  of  turns  in  the  secondary  coil  the  greater  the 
resultant  voltage  across  it. 

121.  By  placing  a  core  of  soft  iron  in  the  primary  coil 
we  very  greatly  increase  the  total  number  of  magnetic 
lines  of  force  induced  by  the  primary  current  (vide 
paragraph  117),  and  therefore,  when  the  primary  circuit 
is  broken,  we  get  a  greater  change  in  the  number  of 
magnetic  lines  of  force  passing  through  the  secondary 
coil,  and  assuming  that  the  time  taken  for  the  magnetism 
to  die  down  is  the  same  as  before,  we  get  a  greater 
rate  of  change  of  magnetic  lines  passing  through  the 
secondary,  and  theref ore  _a ^higher  voltage   is  induced 
across  it. 

122.  Further,  by  using  a  very  fine  wire  we  can  wind 
a  very  large  number  of  turns  on  to  the  secondary  coil, 
thereby   still   further   increasing   the    voltage   induced 
across  the  secondary. 

123.  By  designing  a  coil  on  these  principles,  it  is 
possible  to  obtain  voltages  of  30,000  volts  or  more, 
using  only  a  small  accumulator  battery  giving  4  or  6 
volts  in  the  primary  circuit. 

124.  We  may  now  describe  how  an  induction  coil  is 
actually  made,  and  the  means  by  which  it  can  give  auto- 
matically a  continuous  stream  of  high- voltage  impulses, 
or  sparks,  when  a  low-voltage  battery  is  applied  to  its 
primary  terminals. 

The  mechanical  construction  is  shown  in  section 
in  Fig.  22,  and  the  electrical  connections  are  shown 
diagrammatically  in  Fig.  23. 

125.  The  secondary  coil  A  is  wound  with  about  5000 
turns  of  fine  wire  on  an  ebonite  bobbin  B,  the  bobbin 
having  a  hole  through  the  middle  sufficiently  large  to 


36 


WIRELESS  TELEGRAPHY 


FIG.  22. 


take  the  primary  coil  with  its  iron  core,  the  two  ends 

of  the  secondary  coil 
*fj  M  are  brought  one  to 
each  of  the  terminals 
E,  E,  which  are  called 
the  "  high-tension  " 
terminals  of  the  in- 
duction coil. 

126.  The  iron  core 
C  is  made  of  a  bundle 
of  soft  iron  wire, 
bound  together  with 
cotton  tape,  and 
round  this  core  is 
wound  the  primary 

winding  D,  consisting  of  about  50  turns  of  fairly  thick 

wire,  through  which  the  current  from  the  primary  battery 

has  to  pass  in  order 

to  magnetise  the  iron 

core. 

127.  One    end    of 

this  coil  is   taken 

straight  to  the  posi- 
tive terminal  of  the 

battery   F,    through 

the  manipulating  key 

G.    The  other  end  of 

the  coil,  however, 

instead  of  being  con- 
nected straight  to  the 

negative  terminal  of 

the  battery  F,  is  connected  to  the  spring  arm  or  trembler 

blade  H  of  the  contact  breaker  K. 


WIRELESS  TELEGRAPHY  37 

128.  This  trembler  blade  carries  on  its  side,  which  is 
nearer  to  the  core  C,  a  small  piece  of  soft  iron  L,  and  on 
its  other  side  a  platinum  contact  M.     Another  platinum 
contact  N    is   carried  on    an   adjusting  screw   0  by  a 
brass  bracket  P,  m  such  a  way  that  it  comes  immediately 
opposite  the  contact  M,  the  spring  of  the  trembler  blade 
being  adjusted  so  that  normally  the  two  contacts  M 
and    N    are  making   contact.     The    brass  bracket   is 
connected  to  the  negative  side  of  the  battery  F. 

129.  The  action  of  the  coil  can  best  be  followed  by 
referring  to  the  diagram  of  connections  m  Fig.  23.    The 
contacts  M  and  N  being  in  contact,  if  the  arm  of  the 
manipulating  key  G  is  depressed,  the  electrical  circuit 
through  the  primary  coil  is  completed  and  a  current 
\\  ill  flow  from  the  positive  side  of  the  battery  F,  through 
the  manipulating  key  G,  through  the  coil  D,  through  the 
trembler  blade  H,  through  the  contacts  M  and  N,  through 
the  bracket  P  (Fig.  22),  and  back  to  the  battery  F. 

130T~The  effect  of  the  current  passing  through  the 
coil  D  is  to  magnetise  the  iron  core  C,  and  the  first 
effect  of  this  magnetisation  is  to  induce  a  voltage  across 
the  secondary  coil  of  wire.  This  high  voltage,  however, 
is  only  a  momentary  impulse,  for  it  depends,  as  already 
stated,  upon  the  rate  of  change  in  the  number  of  magnetic 
lines  of  force  passing  through  the  secondary  coil,  so  that 
as  soon  as  the  iron  core  is  fully  magnetised  by  the 
primary  current,  the  change  in  the  number  of  magnetic 
lines  ceases,  and  then-lore  t  he  voltage  across  the  secondary 
falls  to  zero. 

131.  If,  however,  the  primary  current  flowing  round 
the  iron  is  interrupted,  the  iron  core  becomes  demagnet- 
ised, and  there  is  again  a  rapid  change  in  the  number  of 
magnetic  lines  of  force  passing  through  the  secondary 


38  WIRELESS  TELEGRAPHY 

coil,  and  we  get  a  second  high-voltage  impulse  across  the 
secondary  coil. 

132.  Now  this  interruption  of  the  primary  circuit  is 
effected  automatically  by  the  contact  breaker,  for  as 
soon  as  the  iron  core  becomes  magnetised  it  attracts  the 
piece  of  iron  L  (Fig.  23),  which,  as  already  explained,  is 
fixed  to  the  trembler  blade,  carrying  the   contact  M, 
thus  separating  the  contact  M  from  the  contact   N, 
and  interrupting  the  primary  circuit. 

133.  As  soon  as  the  circuit  is  thus  broken  the  iron 
core  C  ceases  to  be  a  magnet,  and  therefore  ceases  to 
attract  the  piece  of  iron  L,  allows  it  to  fly  back  to  its 
original  position,  and  the  primary  circuit  is  again  com- 
pleted through  the  contacts  M  and  N  coming  together 
again. 

The   same  cycle   of  events  repeats  itself  in  rapid 
succession  so  long  as  the  manipulating  key  G  is  kept 


134.  The  resulting  effect  in  the  secondary  coil  is 
therefore    a    corresponding   number   of    high  -  voltage 
impulses  across  the  coil,  one  impulse  being  induced  when 
the  magnetism  in  the  iron  grows  owing  to  the  primary 
current  passing  around  it,  and  a  second  impulse  being 
induced  in  the  opposite  direction  when  the  magnetism  of 
the  iron  collapses  owing  to  the  primary  current  ceasing 
to  pass  around  it. 

135.  As  a  matter  of  fact,  the  magnetism  in  the  iron 
grows  comparatively  slowly  owing  to  the  inductance  of 
the  winding  (vide  paragraphs  61  and  66)  as  compared 
with  the  rate  at  which  the  magnetism  collapses  on 
breaking  the  circuit,  and   as  the  voltage  across  the 
secondary  coil  is  proportional  to  the  rate  of  change  of 
magnetic  lines  of  force,  we  get  a  very  much  bigger 


WIRELESS  TELEGRAPHY 


39 


voltage  during  the  collapse  of  the  magnetism  than 
during  the  growth  of  the  magnetism  ;  that  is  to  say, 
we  get  a  higher  voltage  when  the  primary  circuit  is 
interrupted  than  when  it  is  completed. 

136.  Fig.  24  shows  diagrammatically  the  voltage 
induced  across  the  secondary  of  an  induction  coil.  The 
upper  part  of  the  curve  shows  the  voltage  impulses  due 


FIG.  24. 

to  the  making  of  the  primary  circuit,  and  the  lower  part 
of  the  curve  shows  the  voltage  impulses  due  to  the 
breaking  of  the  primary  circuit. 

137.  From  A  to  B  the  magnetism  in  the  iron  core  is 
growing  comparatively  slowly,  and  the  voltage  induced 
across  the  secondary  only  rises  to  about  1000  volts. 

At  the  point  B  the  primary  circuit  is  broken,  and  a 
very  high  voltage,  perhaps  about  20,000  volts,  is  induced 
across  the  secondary  in  the  opposite  direction,  owing  to 
the  very  rapid  collapse  of  the  magnetism  in  the  iron. 


40  WIRELESS  TELEGRAPHY 

138.  Another  important  part  of  the  induction  coil  is 
the  condenser,  which  is  connected  across  the  contact 
breaker  K,  shown  at  Q  in  Figs.  22  and  23. 

Owing  to  the  inductance  of  the  primary  winding  when 
the  current  is  suddenly  interrupted  at  the  contacts  M 
and  N,  a  high  voltage  is  generated  in  the  primary  coil 
in  the  opposite  direction  to  the  applied  voltage.  This 
voltage  is  sufficient  to  cause  an  arc  to  form  between  the 
contacts  M  and  N  which  interferes  with  the  rapid  inter- 
ruption of  the  current  necessary  to  produce  a  high- 
voltage  impulse  in  the  secondary,  and  in  addition  causes 
the  contacts  to  fuse  together  and  stick. 

139.  By  placing  a  dondenser  across  these  contacts  the 
energy  liberated  when  the  current  is  interrupted  owing 
to  the  inductance  of  the  circuit  (vide  paragraph  62)  ex- 
pends itself  in  charging  up  the  condenser,  thus  reducing 
the  arc  to  a  minimnni  and  preventing  the  fusing  of  the 
contacts. 

By  following  the  diagram  of  connections,  Fig.  23, 
it  will  be  seen  that  the  contacts,  when  closed,  form  a 
short-circuit  to  the  condenser,  thus  allowing  the  con- 
denser to  discharge  itself  through  these  contacts  when 
they  fly  together  again,  if  it  has  not  already  discharged 
itself  through  the  primary  winding  of  the  induction  coil. 

PRODUCTION  OF  ELECTRICITY  BY  CHEMICAL  ACTION 

140.  A  Cell  is  an  apparatus  for  producing  a  current  of 
electricity  by  chemical  action. 

141.  A  Battery  consists  of  a  number  of  cells  joined 
together  either  in  parallel  or  series. 

142.  A  cell  usually  consists  of  two  dissimilar  metals, 
such  as  copper  and  zinc,  immersed  in  a  solution  of  acid 
or  salt,  as  shown  in  Fig.  25. 


WIRELESS  TELEGRAPHY 


41 


—        Z//TC 


Chemical  action  is  set  up  by  the  acid  attacking 
the  zinc,  and  the  energy  liberated  by  the  dissolving 
of  the  zinc  appears  in  the  form  of  electric  potential 
on  the  submerged  surface  of  the  zinc  plate. 

There  is,  however,  practically  no  chemical  action 
set  up  on  the  copper  plate,  and  therefore  no  electric 
potential  is  produced  on  the  surface  of  the  copper  plate, 
with  the  result  that  the  sub- 
merged portion  of  the  zinc  plate 
is  at  a  higher  electric  potential 
than  the  submerged  portion  of 
the  copper  plate,  and  therefore 
below  the  surface  of  the  liquid 
the  zinc  can  be  regarded  as  of 
positive  potential,  and  the  copper 
as  negative. 

143.  When  the  two   ends  of 
the  plates  which  are  above  the 
liquid  are  connected  together  by 

a  conductor,  the  current  will  flow  from  the  zinc  plate  to 
the  copper  plate  under  the  liquid,  and  from  the  copper 
plate  to  the  zinc  plate  above  the  liquid. 

144.  For  this  reason  the  terminal  which  is  joined  to 
the  copper  plate  is  called  the  positive  terminal  of  the  cell, 
and  the  terminal  which  is  joined  to  the  zinc  plate  is 
called  the  negative  terminal  of  the  cell. 

145.  Cells  of  this  nature  are  called  Primary  Cells,  and 
a  battery  consisting  of  two  or  more  of  such  cells  properly 
joined  together  is  called  a  "  Primary  Battery." 

ACCUMULATORS 

146.  An  Accumulator  is  a  cell  in  which  the  two 
plates  are  made  of  such  materials  that  when  a  current  of 


FIG.  25. 


42  WIRELESS  TELEGRAPHY 

electricity  is  passed  through  them  from  some  outside 
source  in  a  certain  direction,  chemical  actions  are  set  up 
between  the  plates  and  the  electrolyte  surrounding  them, 
thus  altering  the  chemical  composition  of  the  materials 
of  which  the  plates  were  made. 

147.  This  is  known  as  "  charging  "  the  accumulator, 
and  the  current  which  is  passed  through  the  accumulator 
is  known  as  the  charging  current. 

148.  On  disconnecting  the  source  of  the  charging 
current,  and  connecting  the  two  plates  of  the  accumu- 
lator together  with  a  conductor,  the  cell  will  act  in  the 
same  way  as  a  primary  cell,  the  chemical  composition  of 
the  plates  will  start  to  return  to  its  original  state,  and  a 
current  of  electricity  will  pass  from  the  cell  through  the 
conductor  in  the   opposite   direction  to   the   charging 
current. 

149.  Such   cells   are    called    "  Secondary   cells "    or 
"  accumulators,"  and  a  battery,  consisting  of   two  or 
more  of  such  cells  properly  joined  together,  is  called 
an   "  Accumulator  Battery,"  "  Secondary  Battery,"  or 
"  Storage  Battery." 


DIAGRAMS  OF  CONNECTIONS 

150.  For  convenience  in  illustrating  graphically  the 
connections  of  an  electrical  circuit,  certain  symbols  are 
used  to  denote  the  particular  predominant  property 
which  that  part  of  the  circuit  possesses.  When  several 
of  these  symbols  are  used  to  illustrate  certain  con- 
nections, it  is  known  as  a  "  Diagram  of  Connections." 

At  the  beginning  of  this  book  we  give  a  number 
of  these  symbols,  and  certain  variations  of  them  which 
are  most  commonly  met  with. 


WIRELESS  TELEGRAPHY  43 

THE  PRINCIPLES  OF  WAVE  MOTION 

151.  Before  showing  how  the  principles  of  electricity 
and  magnetism  are  applied  to  wireless  telegraphy,  we 
must  first  explain  the  principles  of  wave  motion  on 
which  the  science  of  wireless  telegraphy  is  founded. 

152  Let  us  make  a  simple  experiment  to  illustrate 
these  principles. 

In  a  pool  of  water,  and  at  opposite  sides  of  it,  two 
pieces  of  wood  are  floating.  If  we  strike  one  of  these 
pieces  of  wood  with  a  hammer,  or  in  any  other  way  cause 
it  to  disturb  the  water,  it  will  be  observed  that  a  number 
of  ripples  or  waves  are  sent  out  in  all  directions.  Follow 
these  waves  until  they  reach  the  piece  of  wood  at  the  far 
side  of  the  pool  and  it  will  be  observed  that  this  second 
piece  of  wood  is  set  in  motion  by  the  waves. 

153.  This  is  analogous  to  what  occurs  between  two 
wireless  stations.  The  piece  of  wood  that  is  struck  with 
a  hammer  corresponds  to  the  transmitting  station,  the 
water  to  the  transmitting  medium,  and  the  piece  of  wood 
at  the  far  end  of  the  pool  to  the  receiving  station. 

154  The  substance  through  which,  or  on  the  surface  of 
which,  a  wave  travels  is  called  the  medium. 

PROPERTIES  OF  WAVES 

155.  A  wave  has  the  property  of  propagating  itself 
radially  from  a  given  point.  That  is  to  say,  once  a  wave 
has  been  started  it  travels  in  all  directions  away  from 
the  point  at  which  it  was  started. 

An  illustration  of  this  can  be  seen  by  dropping  a  stone 
into  the  middle  of  a  pool  of  water.  The  displacement 
of  the  water  by  the  stone  starts  a  circle  of  ripples  or 
waves,  which  circle  gets  bigger  and  bigger  until  either  the 
waves  die  out  or  they  reach  the  edge  of  the  pool. 


44  WIRELESS  TELEGRAPHY 

156.  A  wave  has  also  the  property  of  producing  at  any 
point  in  its  path  a  disturbance  in  a  body  suspended  in  the 
medium  similar  to  the  disturbance  which  started  the  wave. 

Thus  if  we  start  a  wave  of  water  on  the  surface  of 
a  pond  by  moving  a  stick  in  it,  the  wave  will  cause  a 
motion  in  another  stick  floating  on  the  surface  of  the 
pond  at  any  point  in  the  path  of  the  wave. 

COMMUNICATION  BY  WAVE-MOTION 

157.  We  may  say,  then,  that  if  we  have  a  means  of 
starting  waves  at  one  point  in  a.  medium,  and  a  means 
of  detecting  the  passage  of  the  waves  at  another  point 
in  the  same  medium,  we  have  a  means  of  communicating 
signals  between  these  two  points. 

In  order  to  communicate  intelligence,  however,  we 
must  be  able  to  distinguish  between  different  signals, 
and  by  means  of  the  Morse  Code  (which  is  given  in  full 
at  the  beginning  of  this  book)  the  number  of  different 
signals  which  it  is  necessary  to  produce  at  a  receiving 
station  to  communicate  intelligence  in  the  form  of  words 
has  been  reduced  to  two,  namely,  the  dot  and  the  dash, 
or  "  short  and  long."  By  different  combinations  of  the 
dot  and  dash  we  can  represent  every  letter  in  the  alpha- 
bet, thus  enabling  us  to  spell  out  letter  for  letter  any 
word  or  sentence  desired. 

158.  As  an  illustration,  let  us  see  how  the  Morse  Code 
could,  for  example,  be  applied  to  the  method  of  com- 
munication described  in  paragraph  153. 

If  we  were  to  fix  above  the  receiving  piece  of  wood 
a  sheet  of  iron,  or  some  other  object,  so  that  every  time 
a  wave  passed  under  it  the  piece  of  wood  knocked 
against  this  object,  we  should  get  a  sound  produced  in 
the  form  of  a  tap. 


WIRELESS  TELEGRAPHY  45 

A  single  blow  from  the  hammer  on  the  transmitting 
piece  of  wood  might  produce  two  or  three  ripples,  which 
would  be  translated  by  the  receiving  piece  of  wood  into 
two  or  three  taps.  Several  blows  in  rapid  succession  on 
the  transmitting  piece  of  wood  would  send  out  perhaps 
a  dozen  ripples  following  one  another.  These  would  be 
translated  by  the  receiving  piece  of  wood  by  a  similar 
number  of  taps,  thus  giving  us  a  ready  means  of  producing 
a  short  or  long  effect  at  the  receiving  end,  and  enabling  us 
to  use  the  Morse  Code  for  communicating  intelligence. 

159.  An    important   point    to    understand   is    that 
although  a  wave  travels  from  one  part  of  a  medium  to 
another,  the  medium  itself  does  not  travel,  and,  except 
for  an  up-and-down  or  to-and-fro  motion  while  a  wave 
is  passing,  remains  where  it  is.     This  can  readily  be 
illustrated  by  placing  something  in  a  pond,  such  as  a 
fishing- float,  which  lies  in  the  water  with  its  top  just 
above  the  surface,   and  starting  a  wave  some  little 
distance  from  the  float.     When  the  wave  passes  the  float, 
the  latter  will  be  seen  to  move  up  and  down,  but  will 
not  be  carried  along  with  the  wave. 

MEASUREMENTS  OF  WAVES 

160.  In  order  to  explain  how  the  properties  of  waves, 
more    especially    of 

electric  waves,  can 
be  utilised  for  the 
purpose  of  com- 
munication, we  must 
know  something 
about  the  different 

qualities  or  measurements  of  waves  and  the  terms  used 
to  describe  them. 


46  WIRELESS  TELEGRAPHY 

161.  The  amplitude  of  a  wave  is  the  distance  from  the 
highest  point  to  the  normal  level,  and  is  usually  denoted 
by  the  Greek  letter  a  (alpha),  as  illustrated  in  Fig.  26. 

162.  The  length  of  a  wave  is  the  distance  from  the 
crest  of  one  wave  to  the  crest  of  the  next,  and  is  usually 
denoted  by  the  Greek  letter  X  (lambda),  as  illustrated 
in  Fig.  26. 

If  we  notice  the  surface  of  a  pond  over  which  a  wave 
is  travelling,  we  see  that  only  part  of  the  wave  is  above 
the  normal  level  of  the  water,  for  there  is  a  corresponding 
depression  between  the  crests.  A  complete  wave  con- 
sists of  the  half  which  is  above  and  the  half  which  is 
below  the  normal  level. 

163.  The  velocity  of  a  wave  or  the  speed  of  radiation 
is  the  distance  a  wave  will  travel  radially  in  one  second. 

Thus,  if  we  start  a  wave  on  the  surface  of  a  pond, 
and  it  takes  one  second  from  the  time  it  was  started  for 
the  circle  of  ripples  to  reach  a  point  on  the  pond  10  feet 
away  from  the  starting-point,  the  velocity  of  the  wave 
is  10  feet  per  second. 

164.  The   frequency  of    a  wave  is    the  number   of 
complete  waves  that  will  pass  a  given  point  in  one 
'second,  or,  in  other  words,  the  rapidity  with  which  one 
wave  follows  another. 

A  good  idea  of  what  is  meant  by  frequency  can  be 
got  by  floating  a  cork  on  the  surface  of  a  pond,  and  after 
starting  a  continuous  succession  of  waves,  or  ripples, 
some  little  distance  away,  counting  the  number  of  times 
the  cork  bobs  up  and  down. 

The  number  of  times  it  does  this  in  a  second  is  the 
frequency  of  the  wave. 

165.  Another  definition  of  frequency  can  be  made  in 
terms  of  the  wave-length  and  the  velocity.     The  explana- 


WIRELESS  TELEGRAPHY  47 

tion  can  be  more  readily  followed  by  referring  to  Fig.  27. 
Imagine  a  continuous  succession  of  waves  following  each 
other,  as  shown  in  Fig.  27  ;  if  we  take  two  points  A  and 
B  to  represent  the  distance  that  any  one  of  these  waves 
will  travel  in  one  second,  the  total  number  of  waves 
included  between  the  points  A  and  B  will  be  the  fre- 
quency of  the  wave,  because  all  of  these  waves  have  to 
pass  the  point  A  in  one  second. 

166.  Now  the  number  of  waves  included  between  the 
points  A  and  B  is  equal  to  the  distance  from  A  to  B 
divided  by  the  length  of  the  wave. 


s\r\/\    /\    /\ 

-    toorxrr    - 

/\     ^     /^ 

/\      /\     /\ 

\/   \/   \/ 

\ 

\y  v  v  \ 

FIG.  27. 

y    \/   \/    \j 

E 

Take  an  example  :  Suppose  the  waves  are  travelling 
at  the  rate  of  100  feet  per  second,  then  the  distance  from 
A  to  B  is  100  feet ;  and  suppose  there  are  ten  waves 
included  between  the  points  A  and  B,  as  shown  in  the 
diagram,  it  follows  that  the  length  of  each  wave  is 
10  feet. 

167.  We  may  say,  then,  that 

Velocity 

Frequency  =  — —       ^— , 
Wave-length 

and  therefore  by  transposing, 

Velocity  =  Frequency  x  Wave-length 


or 


.       .,       Velocity 
Wave-length  =  — 

Frequency. 


48  WIRELESS  TELEGRAPHY 

PRESSURE   WAVES 

168.  The   waves   we   have   been   considering   travel 
along  the  surface  of  water,  but  another  kind  of  wave  can 
be  formed  which  travels  through  the  body  of  a  substance. 
Such  waves  are  called  "  Pressure  Waves  "  and  obey  the 
following  laws. 

169.  (1)  A  pressure  wave  travels  at  a  definite  speed 
depending  on  the  substance  or  medium,  and  the  speed 
in  a  given  medium  remains  -the  same  no  matter  how 
big  or  how  small  the  waves  may  be. 

Generally  speaking,  the  speed  or  velocity  is  greater 
the  greater  the  elasticity  l  of  the  substance. 

Thus  in  air  the  velocity  is  about  1090  feet  per  second, 
in  water  it  is  4700  feet  per  second,  and  in  steel  it  is 
16,400  feet  per  second. 

170.  (2)  The  amplitude  of  the  wave  very  rapidly  gets 
smaller  as  the  wave  gets  farther  from  its  starting-point, 
until,  if  given  sufficient  room,  it  finally  dies  out  alto- 
gether ;   in  other  words,  the  amplitude  decreases  as  the 
distance  from  the  starting-point  increases. 

171.  (3)  The  length  of  the  wave  remains  the  same  no 
matter  how  far  it  is  from  its  starting-point ;    in  other 
words,  the  wave-length,  once  started,  remains  constant, 
and  is  quite  independent  of  the  amplitude. 

AETHER  WAVES 

172.  In  order  to  explain  the  phenomena  of  light, 
radiant  heat,  and  electric  waves,  physicists  have  imagined 
a  substance  or  medium  called  the  "  aether,"  and  waves 
similar  to  the  pressure  waves  we  have  just  been  con- 

1  The  elasticity  of  a  substance  is  the  force  which  must  be  applied 
to  a  given  length  of  unit  cross-section  in  order  to  double  this  length 
It  must  not  be  confused  with  the  term  "  flexibility  "  used  in  par.  30. 


WIRELESS  TELEGRAPHY  49 

sidering  produce  rays  having  different  properties,  accord- 
ing to  the  wave-length. 

173.  The    shortest    wave-lengths    known    produce 
X-rays,  which  have  the  property  of  passing  through 
many  bodies  that  are  impervious  to  light  rays,  and  of 
causing  chemical  action  on  photographic  plates.     The 
next  in  length  produce  actinic  rays,  causing  chemical 
action  similar  to  those  produced  by  X-rays.     Then  light 
rays,  which  act  on  the  eyes,  producing  the  effect  of 
vision  ;   and  heat  rays  which   produce  the    effect    of 
warmth  ;  and,  finally,  "  electric  "  rays,  which  will  pro- 
duce electric  currents  in  conductors,  and  which  are  used 
in  Wireless  Telegraphy. 

The  following  is  a  table  of  some  of  these  wave- 
lengths 

X-rays,  about  2-5  millionths  of  an  inch.1" 

Actinic  rays  of  maximum  intensity,  10  millionths  of 
an  inch. 

Light  rays,  from  10  to  18  millionths  of  an  inch. 

Heat  rays  of  maximum  intensity,  about  15  millionths 
of  an  inch. 

Electric  rays,  shortest  measured,  0-24  inch  ;  used  in 
wireless  telegraphy,  300  feet  to  50,000  feet. 

174.  All  these  waves  obey  the  laws  stated  for  pressure 
waves  (paragraphs  169  to  171),  and  have  the  properties 
explained  in  paragraphs  155  and  156,  and,  since  they  all 
travel  through  the  same  medium,  the  velocity  of  all  of 
them  is  that  of  Light — namely,  300,000,000  metres,  or 
about  1,000,000,000  feet  per  second,  equal  to  186,000 
miles  per  second. 

175.  The  velocity  of  aether  waves  is  thus  seen  to  be 

1  The  one-thousandth  part  of  the  thickness  of  a  cigarette  paper 
is  about  one- millionth  of  an  inch. 


50  WIRELESS  TELEGRAPHY 

far,  greater  than  that  of  air  waves.  It  is  for  this  reason 
that,  as  light  travels  in  the  form  of  aether  waves,  and 
sound  travels  in  the  form  of  air  waves,  if  we  watch  a 
battleship  from  a  distance  firing  guns,  we  see  the  flash  of 
5he  gun  long  before  we  hear  the  report. 

176.  Caution   must  be   exercised   when   the   effects 
produced  by  surface  waves  are  used  to  explain  the 
phenomena  of  Wireless  Telegraphy,  Sound,  or  other 
effects  transmitted  by  pressure-wave  radiation,  because 
surface  waves  do  not  follow  altogether  the  laws  governing 
pressure  waves.    Thus  they  do  not  follow  Law  No.  1, 
for  the  velocity  of  the  surface  waves  on  water  depends  on 
the  wave-length  and  amplitude,  i.e.  big  waves  travel 
faster  than   small  ones  ;    further,  they  do  not  follow 
Law  No.  2,  for  the  amplitude  of  surface  waves  is  not 
independent  of  the  wave-length,  thus  if  a  surface  wave 
of  definite  length  be  started,  it  will  be  found  that  its 
length  will  increase  as  its  amplitude  decreases. 

In  general  we  may  make  the  following  deductions 
regarding  the  effects  produced  by  wave  motion. 

177.  (1)  The  nature  of  the  effect  produced  by  a  wave, 
or  a  series  of  waves  in  a  given  medium,  on  the  senses, 
or  on  other  matter,  depends  upon  the  frequency  of  the 
waves. 

Thus,  if  the  frequency  of  waves  travelling  in  aether 
lies  between  about  1200  billions  and  660  billions,  they 
will  produce  an  effect  on  the  eyes  known  as  light,  and 
the  various  frequencies  between  those  limits  will  produce 
various  colours. 

178.  (2)  The  strength  of  the  effect  produced  by  a  wave 
depends  upon  the  amplitude  of  the  wave,  and  since  the 
amplitude  of  the  wave  gets  smaller  as  the  wave  gets 
farther  from  its  starting-point,  the  effect  produced  by  a 


WIRELESS  TELEGRAPHY  51 

wave  will  be  weaker  as  the  distance  from  its  starting-point 
is  increased. 

Thus,  taking  a  lighted  candle  as  the  starting-point 
of  a  number  of  light  waves,  it  will  be  observed  that  the 
strength  of  the  light  it  produces  on,  say,  a  sheet  of  paper 
is  very  rapidly  reduced  as  the  distance  between  the 
paper  and  the  candle  is  increased. 

COMMUNICATION  BY  MEANS  OF  AETHER  WAVES 

179.  In  paragraph  152  we  showed  how,  by  means  of 
waves  on  the  surface  of  a  pond,  we  could  communicate 
signals  from  one  point  to  another,  but  the  method  there 
described  would  be  exceedingly  slow,  and  would,  for 
many  other  reasons,  be  quite  impracticable. 

Since  Aether  Waves  also  possess  the  properties 
mentioned  in  paragraphs  155  and  156,  it  is  obvious  that 
these  waves  can  be  used  in  a  similar  manner  for  the 
purpose  of  communicating  signals  from  one  point  to 
another  (vide  paragraph  157),  with  the  great  advantage 
that,  the  speed  of  radiation  being  186,000  miles  per 
second,  communication  from  one  point  to  another  will 
be  practically  instantaneous. 

180.  Communication    has,    for   many  years    before 
"  Wireless  Telegraphy  "  was  thought  of,  been  carried 
out  by  means  of  Aether  Waves  in  the  form  of  light  waves, 
by  using  searchlights,  heliographs,  and  similar  apparatus, 
but  this  method  has  the  disadvantage  that  the  range  is 
small  and  intervening  objects  interrupt  communication. 

The  discovery  leading  to  Marconi's  great  invention 
of  Wireless  Telegraphy  was  made  by  Hertz.  Hertz 
first  experimentally  proved  the  existence  of  electric 
waves  and  indicated  how  they  could  be  produced  by 


52  WIRELESS  TELEGRAPHY 

electrical  means.     For  this  reason  they  are  sometimes 
known  as  Hertzian  waves. 

It  should  be  understood,  though,  that  Hertz  only 
demonstrated  the  existence  of  these  waves,  and  did  not 
in  any  way  attempt  to  utilise  them  as  a  means  of  com- 
munication. 

181.  The  advantages  which  electric  waves  have  over 
light  waves  for  communication  can  be  briefly  stated  as 
follows  : 

182.  (1)  They  will  pass  through,  or  over,  intervening 
objects  which  are  impervious  to  light  waves,  and  therefore 
these  objects  will  not  interrupt  communication. 

183.  (2)  They  will  follow  the  curvature  of  the  earth, 
and  therefore  the  range  of  communication  can  be  in- 
creased beyond  the  limits  of  the  horizon,  whereas  in  the 
case  of  light  waves  it  is  necessary  that  the  point  from 
which  a  ray  of  light  is  being  received  is  above  the  horizon. 


PRODUCTION   OF  WAVES 

184.  We  have  already  stated  that  waves  formed  on 
the  surface  of  a  body  do  not  follow  exactly  the  same  laws 
as  pressure  waves,  but  the  analogy  of  the  wave  produced 
on  the  surface  of  a  pool  is  useful  in  explaining  how  a 
pressure  wave  is  produced. 

•Let  us  first  understand  clearly  what  the  difference 
is  between  a  wave  produced  on  the  surface  of  a  body 
and  a  wave  produced  through  the  substance  of  a  body. 

185.  The  wave  on  the  surface  of  a  pool  depends  for 
its  existence  upon  a  difference  in  the  height  of  adjacent 
particles  of  water  above  or  below  the  normal  level  of 
the  water.     It  may  therefore  be  called  a  height  wave. 

186.  The  wave  produced  through  the  substance  of  a 


WIRELESS  TELEGRAPHY 


53 


body,  oil  the  other  hand,  depends  for  its  existence  upon 
a  difference  in  the  pressure  of  adjacent  particles  of  the 
substance  through  which  it  is  travelling  above  or  below 
the  normal  pressure  of  that  substance.  It  is  therefore 
called  a  pressure  wave. 

PRODUCTION  OF  HEIGHT  WAVES 

187.  It  is  not  generally  known  why  a  height  wave  is^ 
produced  on  the  surface  of  a  pool  when  a  stone  is  dropped 
into  it,  and  therefore  an  explanation  of  it  will  be  useful 
before  describing  how  a  pres- 
sure wave  can  be  started. 

188.  It  is  evident  that  as 
we  fill  up  a  pond  we  raise  the 
height  of  the  surface  of  the 
water  in  that  pond.    It  does 
not  matter  with  what  material 
we  fill  the  pond  up,  whether  it 
be  water    or    stones   or  any 
other  matter,  the  effect  is  the 
same,  namely,  the  height  of 

the  surface  of  the  water  is  increased. 

189.  It  follows,  then,  that  if  we  drop  only  a  single 
stone  into  a  pool,  we  increase  the  height  of  the  surface  of 
that  pool,  even  though  it  be  ever  so  slightly. 

Owing  to  the  inertia  of  the  water,  however,  the 
height  of  the  water  is  not  immediately  increased  over 
the  whole  surface  of  the  pool,  for  the  inertia  of  the  water 
tends  to  prevent  it  from  rising.  The  result  is,  that  when 
the  stone  is  plunged  into  a  pool,  the  water  that  is  dis- 
placed by  the  stone  forms  in  a  heap  all  round  the  stone, 
as  shown  in  Fig.  28.  A  height  wave  is  thus  started  on 
the  surface  of  the  water,  and  travels  radially  like  an  ever- 


Fio.  28. 


54  WIRELESS  TELEGRAPHY 

expanding  circle  with  the  point  where  the  stone  entered' 
the  water  as  its  centre. 

190.  The  size  of  the  pool  makes  no  difference  to  the 
production  of  the  wave,  for  it  is  just  as  easy  to  start  a 
wave  in  the  middle  of  the  ocean  as  it  is  in  a  pool  of  water, 
for  the  effect  does  not  depend  upon  the  inertia  of  the 
whole  mass  of  water,  the  inertia  of  the  water  immediately 
surrounding  the  stone  being  sufficient. 

PRODUCTION  OF  PRESSURE  WAVES 

191.  We  may  take  a  very  similar  experiment  to  ex- 
plain the  production  of  a  pressure  wave  in  the  air,  but  it 

must  be  remembered 
in  this  experiment 
that  instead  of  start- 
ing a  circle  of  maxi- 
mum height  on  the 
surface  of  a  body 
we  start  a  circle,  or 
rather  a  sphere  of 
maximum  pressure, 
in  the  substance  of 
the  body. 

Flo  29  Let    us    imagine 

a  closed  chamber  full 

of  air  with  an  opening  at  the  bottom  through  which 
we  can  pump  some  water,  as  shown  in  Fig.  29. 

192.  It  is  well  known  that  if  we  pump  anything  into 
the  chamber  we  increase  the  pressure  on  the  air  inside  it. 
It  does  not  matter  what  we  pump  in,  whether  it  be  water 
or  air,  the  effect  is  the  same,  namely,  the  pressure  is 
increased. 

193.  We  will  suppose,  for  the  purpose  of  explanation, 


WIRELESS  TELEGRAPHY  55 

that  the  chamber  is  full  of  air  at  normal  pressure  and 
that  we  increase  the  pressure  of  this  air  by  pumping 
water  into  the  chamber. 

194.  If,    then,    we   suddenly   force    water   into    the 
chamber,  owing  to  the  inertia  of  the  air  we  momentarily 
only  increase  the  pressure  in  the  air  immediately  above 
the  surface  of  the  water,  or  in  other  words,  the  air  that  is 
displaced  by  the  water  forms  a  pressure  heap  which 
travels  forward  in  the  form  of  a  pressure  wave  through 
the  substance  of  the  air 

195.  It  must  be  understood,  though,  that  the  air 
itself  does  not  travel,  but  only  the  pressure  of  the  air 
travels,  just  as  tho  water  that  is  displaced  by  the  stone 
does  not  travel,  but  only  the  height  of  the  water  travels. 

196  It  must  be  further  understood  that  the  formation 
of  a  pressure  heap  of  air  in  the  chamber  does  not  depend 
on  the  inertia  of  the  whole  of  the  air  in  that  chamber, 
the  inertia  of  the  air  immediately  above  the  water  being 
sufficient,  and  therefore  just  as  the  height  wave  can  be 
produced  as  easily  in  the  middle  of  the  ocean  as  in  the 
middle  of  a  pond,  so  can  a  pressure  wave  of  air  be 
produced  in  the  open  atmosphere  just  as  easily  as  it  can 
be  in  a  closed  vessel,  it  being  only  necessary  to  displace 
some  of  the  air  at  a  given  point  to  start  a  wave. 

197.  Since  the  air  is  invisible  it  is  impossible  to  see 
these  pressure  waves,  and  they  can  only  be  imagined  ; 
but  a  simple  experiment  can  be  made  in  which  a  pressure 
wave  can  be  actually  seen,  and  which  compares  very 
closely  to  the  experiment  explained  above. 

198.  Let  us  make  up  a  very  long  spiral  spring  out  of 
fine  steel  wire,  as  shown  at  A  in  Fig.  30,  and  support  it 
at  intervals  along  its  whole  length  by  threads,  so  as  to 
allow  it  a  greater  freedom  of  motion  than  if  we  laid  it  on 


56  WIRELESS  TELEGRAPHY 

a  table  where  any  motion  would  have  to  overcome  /he 
friction  of  the  table. 

199.  If  now  we  strike  one  end  of  this  spiral  a  sharp 
tap  with  a  hammer,  it  will  be  observed  that,  at  the 
moment  of  impact,  only  that  part  of  the  spring'  im- 
mediately in  front  of  the  hammer  will  be  compressed, 
while  the  rest  of  the  spring  remains  as  it  was. 

200.  This  compression  will  be  seen  to  travel  along  the 
whole  length  of  the  spiral,  like  a  pressure  wave,  leaving 
that  part  of  the  spiral  between  it  and  the  hammer  in  its 
normal  state  after  it  has  passed  ;  thus  at  the  moment  of 

vJlw/Jww^^  A 


<=/pOWPSWWWWM/^^  B 

c,_XpQM/VVVW\MA/WWVWW^  C 

FIG.  30. 

impact  the  spring  will  take  the  form  as  shown  at  B  in 
Fig.  30,  and  when  the  wave  has  travelled  half  the  length 
of  the  spiral,  tb3  spring  will  take  the  form  as  shown  at 
C  in  Fig.  30. 

201.  To  carry  out  this  experiment  successfully,  the 
inertia  of  the  spring  must  be  made  big  by  using  a  very 
long  spiral,  say  30  feet  long ;  also  the  wire  forming  the 
spiral  must  be  extremely  fine,  so  as  to  allow  it  to  com- 
press easily  without  exerting  too  great  a  force  against 
the  inertia  of  the  whole  spring,  otherwise  the  effect  will 
be  produced  too  rapidly  for  observation,  and  it  will 
appear  that  the  spiral  is  only  moved  bodily  by  the 
hammer.  A  suitable  spiral  would  be  a  coil  of  fine  steel 


WIRELESS  TELEGRAPHY 


wire,  say  No.  28  gauge,  wound  on  a  coil  say  1  inch  in 
diameter  and  20  feet  or  30  feet  long. 

The  spring  should  be  suspended  by  threads  as  long 
as  possible  and  at  intervals  as  frequent  as  possible. 

202.  In  the  above  experiments  we  have  considered 
that  a  wave  is  produced  by  a  sudden  increase  in  the 
pressure  at  a  given  point.  This,  however,  is  not  strictly 
accurate,  for  a  single  pressure  pulse  ddes  not  produce  a 
complete  wave,  but  only  one-quarter  of  a  wave. 

20.3.  To  produce- a  complete  wave,  it  is  necessary  that 
the  pressure  be  first  in- 
creased above  normal, 
then  reduced  to  normal, 
then  reduced  to  below 
normal,  and  finally 
increased  again  to 
normal. 

204.  An  illustration 
of  this  is  given  in  Fig. 
31,  where  a  long  india- 
rubber  tube  is  shown 
full  of  water.  The 
size  of  the  tube  can  be 
increased  or  decreased 
by  connecting  it 
through  a  .pipe  to  a  pump,  "capable  of  pumping  more 
water  into  the  tube  and  of  sucking  some  of  the  water 
out  of  it,  thus  increasing  and  decreasing  the  size  of 
the  tube. 

The  full  line  in  the  illustration  shows  the  normal 
size  of  the  tube,  and  the  two  dotted  lines  show  the 
maximum  and  minimum  sizes  of  the  tube. 

205.  When  the  tube  has  been  expanded,  reduced  to 


FIG.  si. 


58  WIRELESS  TELEGRAPHY 

normal  size,  contracted  and  increased  to  normal  size,  it 
is  said  to  have  passed  through  one  complete  cycle  of 
operations. 

206.  As  the  size  of  the  tube  is  increased,  the  pressure 
of  the  air  surrounding  it  is  increased  above  normal, 
and  vice  versa  as  the  size  of  the  tube  is  decreased  the 
pressure  of  the  air  surrounding  it  is  decreased  below 
normal. 

Therefore,  if  we  pass  this  tube  through  one  cycle 
of  conditions,  we  shall  produce  in  the  air  surrounding 
it  one  complete  pressure  wave. 

207.  In  Fig.  32  the  same  tube  is  shown  in  different 
stages  of  the  cycle  with  relation  to  time,  assuming  that 
the  time  taken  to  inflate  and  deflate  the  tube  through 
one  cycle  is  one  second.     Thus  at  the  commencement  of 
operations  the  tube  is  at  its  normal  size,  as  shown  at  A. 
At  the  end  of  a  quarter  of  a  second  the  tube  has  been  ex- 
panded to  its  maximum  size,  as  shown  at  B.    At  the  end 
of  half  a  second  the  tube  has  been  reduced  again  to  its 
normal  size,  as  shown  at  C.     At  the  end  of  three-quarters 
of  a  second  the  tube  has  been  contracted  to  its  minimum 
size,  as  shown  at  D,  and  at  the  end  of  one  second  the 
tube  has  once  more   returned  to  its  normal  size,  as 
shown  at  E. 

208.  The  effect  on  the  pressure  of  the  air  surrounding 
the  tube,  with  relation  to  the  time,  can  be  illustrated 
diagrammatically  by  the  curve  drawn  below  the  illustra- 
tion of  the  tube,  where  the  distance  of  the  curve  above 
or  below  the  horizontal  line  represents  the  increase  or 
decrease  in  the  pressure  of  the  air,  or  what  comes  to 
the  same  thing,  the  increase  or  decrease  in  the  size  of 
the  tube  and  the  distance  along  the  horizontal  line 
represents  the  progress  of  time 


WIRELESS  TELEGRAPHY 


59 


It  is  evident  that  the  frequency  of  the  wave  produced 
depends  entirely  upon  the  frequency  of  the  impulses 
producing  it. 


I       COMPLETE        CYCLE 
Fid.  32. 


209.  In  paragraph  167  we  defined  the  relation  between 
the  length  of  the  wave,  the  frequency,  and  the  speed  at 
which  it  was  radiated.  This  relation  will  be  better 


60  WIRELESS  TELEGRAPHY 

understood  by  applying  it  to  the  foregoing  illustration 
of  the  production  of  a  complete  wave. 

210.  Since  it  took  one  second  for  the  tube  to  pass 
through  a  complete  cycle  of  operations,  we  can  say  that 
the  frequency  of  these  operations  was  one  per  second, 
and  since  the  cycle  of  operations  only  produced  one  wave, 
we  may  say  also  that  the  frequency  of  the  wave  was  one 
per  second,  or  in  other  words,  the  wave  was  not  complete 

-until  one  second  after 

— f\    /\    /\    /\    /\     .       its  commencement. 

211.    Since     the 

speed  of  radiation  of 

pressure  in  air  is  roughly  1000  feet  per  second,  it 
follows  that  the  beginning  of  the  wave  will  have 
reached  a  distance  of  1000  feet  from  the  starting- 
point  by  the  time  the  end  of  the  wave  has  just  left 
the  starting-point ;  thus  the  length  of  the  wave  will 
be  1000  feet.  It  will  be  seen  we  shall  get  the  same 
result  by  applying  the  formula — wave-length  =  velocity 
-T- frequency. 

Up  to  the  present  we  have  considered  only  the 
production  of  a  single  wave. 

212.  A  group  of  waves  may  be  defined  as  a  natural 
sequence  of  two  or  more  waves  immediately  following 
one  another    with- 
out any  interval 

between    them,    as       /   \     /  \     f\     r\ 

illustrated    in    Fig.  \  /     \J 

33. 

213.  If  a  group 

of  waves  is  produced  in  such  'a  way  that  each  successive 
wave  has  the  same  amplitude,  as  shown  in  Fig.  33,  the 
waves  are  said  to  be  "  continuous." 


WIRELESS  TELEGRAPHY  61 

214.  If  a  group  of  waves  is  produced  in  such  a  way 
that  the  amplitude  oi  each  successive  wave  is  less  than 
its  predecessor,  as  shown  in  Fig.  34,  the  waves  are  said 
to  be  "  damped." 

215.  A  group  of  waves  is  produced  by  a  series  of 
periodic  displacements  of  the  medium,  which  follow  one 
another  without  an  interval. 

216.  It  is  obvious  from  the  experiments  described  in 
paragraphs  203  to  207  that  if  we  repeat  the  cycle  of 
operations  of  expanding  and  contracting  the  rubber  tube 
periodically;  vve  shall  produce  a  group  of  waves,  and  if 
the  extent   of    these  expansions  and    contractions    is 
maintained,  the  result  will  be  to  produce  a  group  of 
continuous  waves  ;  but  if  the  extent  of  these  expansions 
and  contractions  gradually  gets  smaller  and  smaller  till 
the  tube  finally  comes  to  rest  at  its  normal  size,  the  result 
will  be  to  produce  a  group  of  damped  waves. 

PRODUCTION  OF  ELECTRIC  WAVES 

217.  In  order  to  convey  signals  from  one  point  to 
another  by  means  of  wireless  telegraphy  it  is  necessary 
to  have  an  apparatus  for  producing  electric  waves  at 
one  point,  and  an  apparatus  for  detecting  the  presence 
of  such  waves  at  the  other  point. 

218.  We  have  described  how  pressure  waves  in  the 
air  are  produced  by  the  periodic  displacement  of  the  air 
at  a  given  point  through  a  definite  cycle,  causing  the 
pressure  in  the  air  to  vary  from  normal  pressure  to  a 
positive  pressure,   to  normal  pressure,   to  a  negative 
pressure,  and  finally  to  normal  pressure. 

219.  Electric  waves  in  the  aether  are  produced  by 
the  periodic  displacement  of  the  aether  at  a  given  point 
through  a  similar  cycle. 


C2  WIRELESS  TELEGRAPHY 

220.  These  displacements  of  the  aether  are  produced  by 
electrical  charges  in  what  is  known  as  the  Aerial  Wire. 

For  the  purpose  of  explanation  we  may  regard  the 
aether  as  a  substance  which  exists  in  everything. 

221.  When  we  charge  up  a  condenser,  we  put  the 
dielectric  of  the  condenser  in  a  state  of  strain.    This 
state  of  strain  in  the  dielectric  exerts  a  pressure  on  the 
aether  which  exists  in  the  dielectric,  and  the  pressure 
pulse  thus  produced  will    radiate  through  the  aether 
in  a  similar  way  to  the  radiation  of  the  pressure  pulse 
in  the  air  which  we  described  previously. 

222.  This,  however,  only  produces  one  pulse,  and  to 
produce  a  complete  wave  in  the  aether  we  must  pass  the 
condenser  through  a  complete  cycle  of  operations  by  first 
charging  it  positively,  tjien  discharging  it,  then  charging  it 
negatively,  and  again  discharging  it  (vide  paragraph  203). 

223.  To  produce  a  group  of  waves  we  must  pass  the 
condenser  through  a  series  of  these  cycles  following  each 
other  in  periodic  sequence. 

224.  If  during  each  cycle  the  condenser  is  charged  to 
the  same  extent,  i.e.  to  the  same  voltage,  the  group  of 
waves  produced  will  be  "  continuous,"  but  if  each  suc- 
cessive charge  of  the  condenser  is  weaker  than  the  last, 
the  group  of   waves   produced  in  the  aether  will  be 
"  damped." 

In  this  book  we  shall  only  describe  the  production 
of  damped  waves* 

225.  Suppose  we  suspend  a  length  of  wire  in  the  air 
from  a  mast  and  insulate  it  from  the  earth,  as  shown  in 
Fig.  35,  we  may  regard  the  wire,  the  air,  and  the  earth 
as  forming  a  condenser,  in  which  the  wire  acts  as  one 
plate  of  the  condenser,  the  air  as  the  dielectric,  and  the 
earth  as  the  other  plate. 


WIRELESS  TELEGRAPHY 


63 


226,  And  suppose  we  have  a  way  of  charging 
and  discharging  it  in  rapid  succession  by  means 
of  a  suitable  electrical  generator  G,  first  charging  it 
positively  and  then  negatively,  each  charge  and  dis- 
charge produces  a  pressure  pulse  in  the  aether,  and 
the  four  pulses — positive  charge,  discharge,  negative 
charge,  discharge— form  a  complete  electric  wave,  which 
starts  travelling  into  space  with  the  velocity  of  light, 
namely  300,000,000  metres  per  second. 

227.  Such  a  wire  is  known 
as  the  Aerial  Wire,  and  is 
given  various  shapes,  as  we 
shall  describe  later. 

228.  The  analogy  of  the 
expansion  and  contraction  of- 
the  india-rubber   tube,   de- 
scribed   in    paragraph    204, 
can  be  used  to  explain  the 
action  of  charging  and  dis- 
charging the  aerial  wire. 

229.  Whilst  being  charged, 
a  current  of  electricity  will 
flow  into  the  wire,  just  as  a 

current  of  water  was  made  to  flow  into  the  tube  to 
expand  it.  The  current  will  be  large  at  first  and 
diminish  as  the  aerial  becomes  charged,  until  it  ceases 
to  flow  when  the  aerial  is  fully  charged.  The  instant 
it  has  ceased  to  flow  the  current  will  start  to  flow  in 
the  opposite  direction,  as  the  aerial  discharges  ;  and  so 
on,  the  current  will  flow  backwards  and  forwards,  charg- 
ing and  discharging  the  aerial  through  successive  cycles. 

230.  Such  a  current  of  electricity  is  called  an  oscillat- 
ing current. 


Fio.  35. 


64  WIRELESS  TELEGRAPHY 

231.  It  is  obvious  that  the  frequency  of  the  wave 
produced  in  the  aether  will  depend  entirely  upon  the 
frequency  of  the  oscillations  in  the  aerial  (vide  para- 
graph 208). 

232.  It  follows,  therefore,  that  by  varying  the  fre- 
quency of  the  oscillations  in  the  aerial  we  can  vary  the 
length  of  the  wave  radiated. 

From  the  formula 

Wave-length 


x  requency 

it  can  be  seen  that  the  greater  the  frequency  of  the 
oscillations  the  shorter  the  wave-length. 

233.  The  wave-lengths  usually  employed  for  the  pur- 
pose of  wireless  telegraphy  vary  in  length  from  100 
metres  to  15,000  metres  or  more.     Generally  speaking, 
the  larger   the   power  of    the  station   the   longer  the 
wave-length  employed. 

The  wireless  apparatus  on  ships  and  at  the  shore 
stations  with  which  the  ships  communicate  is  designed 
to  transmit  wave-lengths  of  300  metres  and  of  600 
metres.  Long-distance  stations  use  wave-lengths  vary- 
ing from  1000  to  15,000  metres. 

234.  From   the   formula    quoted   above   it  will   be 
found  that  the  number  of  oscillations  per  second  required 
to  produce  a  wave-length  of  15,000  metres  is  20,000, 
an$  the  number  per  second  required  to  produce  a  wave- 
length of  100  metres  3,000,000. 

235.  Such  oscillations  are  known  as  High-frequency 
or  oscillatory  currents,  to  distinguish  them  from  the 
Low-frequency  or  alternating  currents  of  between  25 
and  1000  per  second  produced  by  ordinary  alternating 
current  dynamos. 


WIRELESS  TELEGRAPHY  65 

PRODUCTION   OF  HIGH  FREQUENCY 
OSCILLATIONS 

236.  There    are   several    ways    of    producing    high- 
frequency   oscillatory   currents.      For  the  present   we 
will  confine  ourselves  to  the  method  known  as  the 
"  spark  "  method. 

237.  If  a  condenser  is  charged  and  then  suddenly 
discharged  by  connecting  its  two  opposite  plates  together 
with  a  conductor,  not  only  does  the  current  flow  from 
the  positively  charged  plate  to  the  negatively  charged 
plate  until  the  plates  are  at  the  same  potential,  but  the 
current  continues  to  flow  in  the  same  direction  on  account 
of  the  inductance  (vide  paragraph  61)  of  the  conductor, 
causing  that  side  of  the  condenser  which  before  was 
negatively  charged  to  become  positively  charged,  and 
vice  versa. 

238.  The  reversed  charge,  however,  does  not  charge 
the  condenser  to  the  same  extent,  i.e.  to  the  same  voltage 
as  the  original  charge,  because  a  certain  amount  of  the 
energy  is  frittered  away  by  the  resistance  of  the  circuit, 
and   a  further  amount  of  energy  is  used   up  in  the 
production  of  pressure  waves. 

239.  The  action  is  then  reversed  with  the  same  effect, 
and  so  on,  each  time  with  less  energy,  until  the  whole 
of  the  energy  originally  in  the  condenser  is  absorbed. 
An  oscillating  current  of  gradually  diminishing  strength 
is  therefore  produced. 

240.  The  action  can  be  illustrated  by  making  the 
experiment  with  the  pendulum  illustrated  in  Fig    36, 
where  the  weight  \V  is  shown  suspended  from  a  fixed 
point  A  by  a  piece  of  string  B. 

241.  If  this  weight  be  displaced  to  the  position  sbowr. 


66 


WIRELESS  TELEGRAPHY 


by  the  dotted  line  marked  Wx  and  then  released,  it 
will  not  immediately  take  up  the  position  W,  but  owing 
to  the  momentum  of  the  weight  will  swing  backwards 
and  forwards  between  the  positions  Vfl  and  W2. 

242.  Owing  to  the  friction  between  the  weight  and  the 
air,  and  also  to  the  fact  that  it  gives  up  some  of  its 
energy  to  the  surrounding  air  in  forming  pressure  waves, 


B\ 


FIG.  36. 

each  swing  will  be  a  little  shorter  than  the  last,  and  it 
will  finally  come  to  rest  at  the  position  W,  after  making 
a  number  of  swings. 

243.  In  this  case  the  number  of  swings  which  take 
place  in  a  second — that  is,  the  frequency  of  the  oscilla- 
tions—can only  be  varied  by  varying  the  length  of  the 
string. 

244.  A   similar   experiment   can    be    made    with   a 
vibrator,  shown  in  Fig,  37,  in  which  there  is  a  flat  steel 


WIRELESS  TELEGRAPHY 


67 


spring  B,  fixed  firmly  at  the  point  A,  and  carrying  a 
weight  W  at  its  other  or  free  end. 

If  the  weight  be  displaced  and  released,  it  will  swing 
or  "  oscillate  "  between  the  position  Wj  and  W2. 

245.  In  this  case  the  number  of  swings  per  second 
will  depend  upon  the  flexibility  of  the  spring  B  and  also 
upon  the  inertia  of  the  weight  W,  and  we  can  therefore 
vary    the    frequency 

either  by  varying  the 

flexibility  of  the  spring 

or    by    varying    the      ill  I        .  .A'WJ'       ^8 

weight. 

246.  It    will    be 
found  that  the  longer 
or  thinner  and  there- 
fore the  greater  the 
flexibility  of  the 
spring,    the   less  the 
number  of  swings  per 

second,  also  the   greater  the    weight  W   the  less    the 
number  of  swings  per  second. 

247.  The  distance  between  the  position  \Vl  and  the 
position  W  is  called  the  maximum  amplitude  of  the 
swing,  and  generally  half  the  distance  between  succes- 
sive positions,  such  as  W3,  W4  in  Fig.  37,  is  called 
the  amplitude. 

248.  Owing  to  friction  of  the  air  and  also  to  the  energy 
radiated  in  the  form  of  pressure  waves,  the  amplitude 
will  start  at  a  maximum  and  gradually  diminish  until 
the  spring  comes  to  rest  in  its  normal  position.     The 
rate  at  which  the  swing  decreases  is  called  the  "damping." 

249.  The  frequency  will,  however,  remain  constant 
quite  independently  of.  the  amplitude  ;  that  is  to  say, 


FIG.  37 


68  WIRELESS  TELEGRAPHY 

in  Fig.  37  the  time  taken  for  the  weight  to  travel  from 
the  position  Wx  to  W2  and  back  again  will  be  exactly 
the  same  as  the  time  it  takes  to  travel  from  the  position 
W3  to  W4  and  back  again. 

250.  To  summarise,  two  matters  have  to  be  con- 
sidered in  connection  with  such  a  vibrator,  viz. : 

(1)  The  frequency,  depending  upon  (1)  the  flexibility 
of  the  spring  and  upon  (2)  the  mass  of  the  weight. 

(2)  The  damping,  depending  upon  the  friction  and 
the  rate  at  which  the  energy  is  radiated. 

OSCILLATORY  CIRCUITS 

251.  A  circuit  in  which  an  oscillating  current  will 
flow  is  called  an  oscillatory  circuit,  and  must  possess  two 
essential  qualities,  namely,  Capacity  and  Inductance. 

It  may  be  compared  with  the  vibrator  described  in 
the  last  article. 

252.  The  properties  of   the  vibrator  which  decide 
the  frequency  of   the  vibrator   are   its   mass  and  its 
springiness. 

We  have  already  explained  in  paragraphs  28  and  62 
that  the  property  of  capacity  is  analogous  to  that  of 
springiness  and  the  property  of  inductance  is  analogous 
to  that  of  mass. 

Similarly,  therefore,  the  properties  of  an  oscillatory 
circuit  which  decide  the  frequency  of  the  oscillating 
currents  that  will  flow  in  it,  and  therefore  the  wave- 
length it  will  produce,  are  its  Inductance  and  its  Capacity, 

253.  Also  the  property  which  tends  to  stop  or  "  damp  " 
the  vibrations  of  the  vibrator  is  friction  and  radiation 
of  energy.     Similarly  the  property  which  tends  to  damp 
the  oscillating  current  in  an  oscillatory  circuit  is  the 
Resistance  of  the  circuit  and  radiation  of  energy. 


WIRELESS  TELEGRAPHY  69 

254.  Obviously  resistance  is  an  undesirable  property, 
as  it  absorbs  energy.     In  every  oscillatory  circuit,  there- 
fore, the  resistance  is  reduced  to  a  minimum  quantity, 
which  is  effected  by  increasing  the  size  of  the  conductor 
and  reducing  its  length  as  much  as  possible  ;   there  is, 
however,  always  a  certain  amount  of  resistance  left. 

255.  As  oscillatory  circuits  are  used  for  the  production 
»f  electric  waves,  it  is  important  to  know  the  relation 
between  the  wa/ve-length  and  the  capacity  and  inductance 
of  the  circuit. 

256.  It  will  be  found  that  as  we  increase  either  the 
value  of  the  capacity  or  of  the  inductance  of  the  circuit,  so 
do  we  decrease  the  frequency  of  the  circuit,  just  as  the 
frequency  of  a  vibrator  is  decreased  by  increasing  either 
its  springiness  or  its  mass. 

257.  The  frequency  of  an  oscillatory  circuit  is  inversely 
proportional  to  the  square  root  of  the  capacity  and  the 
square  root  of  the  inductance.     And  since  the  wave- 
length is  inversely  proportional  to  the  frequency,  it 
follows  that  the  wave-length  produced  is  proportional 
to  the  square  root  of  the  capacity  and  the  inductance. 

258.  This  law  can  be  expressed  as  a  formula,  thus* 

Wave-length  =  ^Capacity  x  v/Inductance, 

or  using  the  symbols  by  which  these  quantities  are 
known, 

fcWCxL* 

259.  This  formula  does  not  define  what  units  are  used 
to  measure  the  different  qualities.     But  if  we  measure 
the  wave-length  in  metres,  the  capacity  in  microfarads, 
and  the  inductance  in  microhenries  the  formula  becomes 


\(m)  =  1885   VQmf)  x  L(mh). 


70  WIRELESS  TELEGRAPHY 

ENERGY  AND  POWER  IN  OSCILLATORY  CIRCUITS 

260.  In  paragraph  178  we  explained  that  the  strength 
of  the  effect  produced  by  a  wave  depends  upon  the 
amplitude  of  the  wave,  and  since  the  amplitude  of  the 
wave  decreases   very  rapidly  as  the  distance  it  has 
travelled  is  increased,  it  follows  that  the  effect  produced 
on  a  receiver  gets  rapidly  weaker  as  the  distance  from 
the  transmitter  is  increased. 

261.  It  is  evident  that  to  get  a  stronger  effect  at  the 
same  distance,  or  to  produce  the  same  effect  at  a  greater 
distance,  we  must  increase  the  amplitude  of  the  wave 
at  the  starting-point. 

262.  It  is  simpler  to  consider  wave- motion  as  a  means 
of  radiating  energy  ;  also  to  consider  the  effect  produced 
by  the  wave  on  what  we  may  call  a  receiver,  as  the 
amount  of  energy  received. 

263.  The  amount  of  energy  received  at  any  point 
must  necessarily  be  very  small  compared  with  the  total 
amount  of  energy  radiated,  because  in  radiating  energy 
'we  spread  that  energy  out  over  a  large  space  ;   thus  the 
farther  the  energy  has  been  radiated  the  greater  the 
space  over  which  it  is  spread. 

264.  Take  the  case  of  a  wave  on  the  surface  of  a  pond. 
At  the  starting-point  the  whole  of  the  energy  in  the 
wave  is  concentrated  in  a  very  small  space,  and  therefore 
a  receiver  in  the  shape  of  a  piece  of  wood  of  comparatively 
small  dimensions  would  receive  the  whole  of  the  energy 
in  that  wave.     If,  however,  we  took  that  same  receiver 
to  a  point  6  feet  away  from  the  starting-point,  that  is 
to  say,  6  feet  away  from  the  transmitter,  by  the  time 
the  wave  reached  it,  it  would  be  spread  over  the  circum- 
ference of  a  circle  12  feet  in  diameter,  and  therefore  only 


WIRELESS  TELEGRAPHY  71 

a  very  small  part  of  the  whole  of  the  wave  would  affect 
the  receiving  piece  of  wood,  or,  in  other  words,  it  would 
only  receive  a  small  part  of  the  energy  in  the  wave. 

265.  It  is  evident  that  the-  farther  we  get  from  the 
starting-point  the  smaller  the  proportion  of  energy 
which  a  given  object  will  receive,  although  the  energy 
in  the  whole  of  the  wave  remains  the  same. 

206.  The  energy  radiated  depends  upon  the  energy 
put  into  the  oscillatory  circuit  producing  the  waves, 
and  it  is  therefore  important  to  know  on  what  factors 
the  energy  in  the  oscillatory  circuit  depends. 

267.  In  the  method  of  exciting  an  oscillatory  circuit 
which  we  are  now  considering,  namely,  the  spark  method, 
the    amount  of    energy    put    into    the    circuit    depends 
upon  the  capacity  of  the  condenser  in  the  circuit,  and  the 
voltage  to  which  it  is  charged. 

268.  The  vibrator  described  in  paragraph  2 14  can  be 
energised  by  applying  an  initial  pressure  to  the  end  of 
the  spring,  thus  bending  the  spring. 

269.  Similarly  an  oscillatory  circuit  is  energised  by 
applying  an  initial  pressure,  or  voltage,  to  the  condenser, 
thus  charging  it  with  electricity. 

270.  The  amount  of  energy  put  into  the  vibrator 
depends  upon  the  flexibility  of  the  spring,  and  the  initial 
pressure  that  is  applied  to  it.     Thus  the  greater  the 
flexibility  of  the  spring  the  greater  the  energy  put  into 
it  by  applying  a  given  pressure.     Also  the  greater  the 
pressure  applied  to  it,  the  greater  the  energy  put  into  a 
spring  of  given  stiffness. 

271.  Similarly   the  amount  of  energy   put  into  an 
oscillatory  circuit  depends  upon   the  capacity   of  the 
condenser  in  the  circuit,  and  the  initial  pressure,  or 
voltage,  to  which  it  is  charged. 


72  WIRELESS  TELEGRAPHY 

272.  A  little  consideration,  however,  will  show  us 
that  the  amount  of  energy  is  not  directly  proportional 
to  the  voltage  to  which  the  condenser  is  charged,  but  is 
proportional  to  the  square  of  that  voltage. 

273.  Taking  again  the  analogous  mechanical  case  of 
the  spring,  it  is  evident  that  if  we  apply  a  force  of, 
say,  1  Ib.  to  a  certain  spring  by  hanging  a  1  Ib.  weight 
to  it,  the  spring  will  be  stretched  to  a  certain  definite 
extent,  as  described  in  paragraph  29. 

274.  Let  us  suppose,  for  the  purpose  of  explanation, 
that  the  spring  is  stretched  1  foot  when  a  force  of  1  Ib. 
is  applied  to  it,  and  let  us  for  the  moment  suppose 
that  throughout  the  process  of  being  stretched  a  force 
of  1  Ibl  is  being  exerted  on  the  spring  by  the  weight, 
then  it  follows  from  the  definition  of  the  mechanical 
unit  of   energy  given  in  paragraph   73  that   1   foot- 
pound of  energy  has  been  expended  on  the  spring. 

275.  Now  the  amount  which   a  given  spring   will 
extend   is  directly  proportional  to   the  force  applied 
to  it,  thus  in  this  case,  if  instead  of  applying  a  force 
of   1   Ib.    to   the   same   spring   we   apply   a   force  of 
2  Ibs.  to  it,  the  spring  in  this  case  will  be  stretched 
2  feet. 

If  for  the  moment  we  again  suppose  that  throughout 
the  process  of  being  stretched  2  feet  a  force  of  2  Ibs.  is 
being  exerted  on  the  spring,  then  it  follows  that  in  this 
case  4  foot-pounds  of  work  have  -  been  expended  on 
the  spring,  because  a  force  of  2  Ibs.  has  been  exerted 
in  moving  a  body  2  feet. 

276.  In  other  words,  although  we  have  only  applied 
twice  the  force  to  the  spring,  we  have  done  four  times  as 
much  work  on  it.     Now  provided  there  are  no  losses  ID 
overcoming  the  friction,  all  the  energy  which  is  done  or 


WIRELESS  TELEGRAPHY  73 

the  spring  is  available  for  use  when  the  spring  is  allowed 
to  contract. 

277.  We  may  say,  therefore,  that  the  energy  stored 
up  in  a  spring  is  proportional  to  the  square  of  the 
force  applied  to  it. 

278.  Now  in  paragraph  69  we  denned  the  expression 
"  flexibility  "  as  being  the  measure  of  .the  amount  by 
which  a  given  spring  would  be  extended  for  a  given 
force  ;    it  follows,  therefore,  that  if  in  the  experiments 
described  above  we  substitute  a  spring  having  twice 
the  flexibility,  we  shall  stretch  this  more  flexible  spring 
a  distance  of  2  feet  by  applying  1  Ib.  of  force  to  it,  thus 
storing  in  it  2  foot-pounds  of  energy.      Further,  we 
shall  stretch  it  a  distance  of  4  feet  by  applying  2  Ibs. 
of   force  to  it,  thus  storing  8  foot-pounds  of   energy 
in  it.     We  may  say,  therefore,  that  the  energy  stored 
up  in  the  spring  is  directly  proportional  to  the  flexibility 
of  the  spring  as  well  as  being  proportional  to  the  square 
of  the  force  applied  to  it. 

279.  Similarly,  if  we  apply  an  E.M.F.  to  a  condenser, 
we  cause  electricity  to  flow  into  the  condenser,  and  the 
quantity  of  electricity   (which  will  correspond  to  the 
amount  by  which  we  stretch  a  spring)  will  be  directly 
proportional  to  the  electromotive  force  applied  to  the 
condenser. 

280.  Thus,  if  the  capacity  of  the  condenser  is  one 
farad  and  an  E.M.F.  of  one  volt  is  applied  to  it,  one 
coulomb  of  electricity  will  be  forced  into  the  condenser 
(vide  paragraph  70),  and  assuming  for  the  moment  that 
this  force  is  being  exerted  throughout  the  process  of 
charging,  it  follows  that  one  joule  of  energy  will  be 
stored  in  the  condenser  (vide  paragraph  74). 

281.  Further,  if  we  double  the  force  applied  to  t!-e 


74  WIRELESS  TELEGRAPHY 

condenser  by  applying  two  volts  instead  of  one,  we  shall 
force  two  coulombs  of  electricity  into  the  same  con- 
denser, and  if  again  we  assume  that  during  the  whole 
process  of  charging,  the  full  force  of  two  volts  is  being 
exerted,  then  in  this  case  it  follows  that  four  joules  of 
energy  will  *be  stored  in  the  condenser,  because  we  get 
two  coulombs  of  electricity  upon  which  a  force  of  two 
volts  has  been  exerted. 

282.  We  may  say,  therefore,  that  the  energy  stored 
up  in  a  condenser  is  directly  proportional  to  the  capacity 
of  the  condenser  and  proportional  to  the  square  of  the 
voltage  applied  to  it. 

283.  Up  to  the  present,  however,  we  have  considered 
for  the  sake  of  simplicity  that  the  force  exerted  is  uniform 
in  the  case  of  the  spring  throughout  the  process  of 
stretching  and  in  the  case  of  the  condenser  throughout 
the  process  of  'charging.     This,  however,  is  in  reality 
not  the  case,  as  will  be  readily  seen  by  analysing  what 
takes  place  during  the  time  the  spring  is  being  stretched. 
Taking,  for  instance,  the  case  of  the  particular  spring 
which  was  stretched  2  feet  by  a  force  of  2  Ibs.    This 
same    spring  was   stretched   1   foot  when  a  force  of 
1  Ib.  was  exerted  on  it.     Similarly  it  will  be  stretched 
only  6  inches  if  a  force  of  half  a  pound  were  exerted  on 
it,  and  so  on. 

284.  Obviously,  therefore,  the  total  force  exerted  on 
the  spring  in  stretching  it  a  distance  of  2  feet  is  not 
the  maximum  force  of  2  Ibs.  but  the  average  of  all 
the  forces  from  0  to  2  Ibs. 

It  will  be  found  that  this  average  force  is  always 
half  the  maximum  force,  assuming  that  the  force  applied 
at  the  commencement  of  the  operation  is  zero. 

285.  Similarly  with  a  condenser  the  average  force 


WIRELESS  TELEGRAPHY  75 

applied  in  charging  it  from  zero  to  a  certain  voltage  will 
be  half  that  maximum  voltage.  Therefore,  in  order  to 
calculate  the  energy  in  the  joules  stored  up  in  a  con- 
denser, we  must  take  half  of  the  product  of  the  capacity 
of  the  condenser  and  the  square  of  the  E.M.F.  to  which 
it  is  charged  in  volts.  This  may  be  stated  as  an 
equation, 

E  =|  CV2, 

where  E  =  energy  in  joules,  C  =capacity  of  condenser 
in  farads  and  V  =  voltage  to  which  the  condenser  is 
charged. 

POWER  IN  OSCILLATORY  CIRCUIT 

286.  In  paragraph  75  we  explained  that  Power  is 
Energy  expended  per  second.     If,  then,  after  charging  a 
condenser  to  a  certain  voltage,  during  which  process  we 
expend  so   much  energy  upon  it,   we   discharge  that 
condenser   we  shall   be   in  a   position  to   recharge   it 
expending  the  same  amount  of  energy  as  before.     It 
follows,   therefore,   that  the   power   used   will   be  the 
product  of  the  energy  expended  during  a  single  charge, 
and  the  number  of  times  per  second  that  it  is  charged. 
We  may  say,  therefore,  that 

Power=-|-CV2xS, 

where  S  is  the  number  of  times  per  second  that  the 
condenser  is  charged. 

OPEN  AND  CLOSED  OSCILLATORY  CIRCUITS 

287.  A  simple  oscillatory  circuit  (vide  paragraph  251) 
is  shown  diagrammatically  in  Fig.  38.      Such  a  circuit 
is  called  a  closed  oscillatory  circuit. 

288.  Another  form  of  osciPHory  circuit  is  shown  in 
Fig. '39,  which  represents  an  aerial  wire  connected  co. 


76 


WIRELESS  TELEGRAPHY 


earth ;  the  aerial  acts  as  one  side  of  the  condenser,  the 
earth  acts  as  the  other  ;  the  wires  forming  the  aerial 
also  form  the  inductance. 

Such  a  circuit  is  called  an  open  oscillatory  circuit. 

289  The  chief  difference  in  the  properties  of  a  closed 
oscillatory  circuit  and  those  of  an  open  oscillatory 
circuit  is  that — 

An  open  oscillatory  circuit  will  produce  waves  having 


FIG.  38. 

a  very  large  amplitude,  and  is  therefore  a  good  radiator, 
whereas  a  closed  oscillatory  circuit  will  only  produce 
waves  of  a  very  small  amplitude,  and  is  therefore  a  very 
bad  radiator. 

290.  The  chief  difference  in  the  composition  of  a  closed 
oscillatory  circuit  and  an  open  oscillatory  circuit  is  that — 

In  a  closed  oscillatory  circuit  the  capacity  and  the 
inductance  are  more  or  less  seDarated  from  one  another, 


WIRELESS  TELEGRAPHY 


77 


all  the  capacity  being  grouped  at  one  point  of  the  circuit 
and  all  the  inductance  at  another  ;  whereas  in  an  open 
oscillatory  circuit  the  capacity  and  inductance  are,  so 
to  speak,  mixed  up  and  distributed  along  the  entire 
length  of  the  circuit.  Thus  the  aerial  wire  itsel-f  is 


FIG   39. 

:act  ng  as  one  plate  of  a  condenser  and  at  the  same  time 
as  an  inductance. 

VARIATION  OF  WAVE-LENGTHS  OF  OPEN 
OSCILLATORY  CIRCUITS 

291.  The  frequency  of  the  aerial  must  be  adjusted 
so  that  it  produces  electric  waves  of  the  desired  length 
(vide  paragraphs  208  and  257),  and  this  can  only  be 


78  WIRELESS  TELEGRAPHY 

done  by  altering  the  capacity  or  the  inductance  of  the 
aerial  circuit. 


TO  INCREASE  THE   WAVE-LENGTH   OF   AN   AERIAL 

292.  The  capacity  of  an  aerial  can  be  increased  by 
increasing  the  number  of  the  wires  forming  it,  and  the 
inductance  of  the  aerial  can  be  increased  by  lengthening 
the  wires.    Thus  the  larger  the  aerial  the  longer  the 
electric  waves  it  will  produce. 

293.  It  would,  however,  be  a  tedious,  and,  in  fact, 
impracticable  operation  to  alter  the  aerial  every  time  it 
was  required  to  alter  the  wave-length,  and  therefore 
another  method  is  adopted  to  increase  the  wave-length 
of  an  aerial. 

294.  If   we   connect   an   inductance   in   series    with 
another  inductance,  the  total  inductance  of  the  circuit 
is   increased.     Therefore,   if   we  insert  an  inductance, 
as  shown  in  Fig.  40,  in  series  with  the  aerial,  we  have 
increased  the  total  inductance  of  the  circuit,  and  thereby 
increased  its  wave-length.     This  method  is  adopted  when 
it  is  desired  to  increase  the  wave-length  of  the  aerial. 

This  added  inductance  tends  to  make  the  open 
oscillatory  circuit  of  the  aerial  into  a  closed  oscillatory 
circuit  (vide  paragraph  290) ;  the  more  inductance  we 
add  the  nearer  do  we  approach  a  closed  oscillatory  circuit, 
for  although  some  of  the  total  inductance  of  the  aerial 
is  still,  so  to  speak,  mixed  up  with  the  capacity,  a  large 
part  of  it  is  separate. 

295.  As  already  stated,  a  closed  oscillatory  circuit 
does  not  radiate  energy  to  any  appreciable  extent  (vide 
paragraph    289) ;     we   therefore   reduce   the   radiating 
properties  of  the  aerial  by  adding  inductance.    There 


WIRELESS  TELEGRAPHY  79 

is  therefore  a  limit  to  the  amount  of  inductance  that 
can  be  so  inserted  into  the  aerial  circuit  without  seriously 
affecting  its  efficiency  as  a  radiator. 

In    practice    it   is   found    that   the   natural   wave- 
.  length   of  an   aerial   can  be   about  doubled  without 


TO   INCREASE       A 

FIG.  40 

seriously  interfering  with  the  radiation  ,  thus  we  have 
a  simple  means  of  controlling  the  wave-length  over  a 
comparatively  large  range. 

TO   REDUCE   THE   WAVE-LENGTH    OF   AN   AERIAL 

296.  If  we  place  a  capacity  in  series  with  another 
capacity,  the  total  capacity,  instead  of  being  increased, 
as  might  at  first  be  imagined,  is  reduced. 


80 


WIRELESS  TELEGRAPHY 


Therefore,  if  we  insert  a  condenser,  as  shown  in 
Fig.  41,  in  series  with  the  aerial,  the  total  capacity  is 
reduced,  and  therefore  the  wave-length  is  also  reduced. 

297.  The  amount  by  which  we  decrease  the  capacity 
depends  upon  the  capacity  of  the  condenser  which  is 


r 


TO   DECREASE       A 

FIG.  41. 

inserted  in  series,  and  it  is  important  to  remember  that 
the  greater  the  capacity  which  is  inserted  in  series  with 
another  capacity,  the  less  is  the  reduction  of  the  total 
capacity ;  that  is  to  say,  by  inserting  a  small  capacity 
in  series  with  the  aerial  we  reduce  the  wave-length  of 
that  aerial  far  more  than  by  inserting  a  large  capacity 
in  series  with  it. 


WIRELESS  TELEGRAPHY 


81 


298.  Similarly,  as  in  the  case  of  adding  .inductance, 
the  insertion  of  a  capacity  in  series  with  the  aerial 
reduces  the  radiation  of  the  aerial,  but  in  practice  it  is 
found  that  the  natural  wave-length'  of  the  aerial  can  be 


SHORT  CIRCUITING 

SWITCH  FOR 
CUTTIN&OUT  THE 
CONDENSER 


FIG.  42. 

about  Halved  by  this  means,  without  seriously  interfering 
with  the  efficiency  of  the  aerial  as  a  radiator. 

299.  It  will  be  seen,  therefore,  that  by  connecting  a 
condenser  or  an  inductance  in  series  with  the  circuit 
the  length  of  the  electric  waves  emitted  by  the  aerial 
can  be  varied  from  nearly  one-half  to  double  the 
natural  wave-length  of  the  aerial  without  seriously 
affecting  its  efficiency  as  a  radiator. 


82  WIRELESS  TELEGRAPHY 

Fig.  42  shows  an  aerial  with  an  adjustable  con- 
denser; and  an  adjustable  inductance  connected  to  it. 
Such  an  aerial  is  capable  of  emitting  waves  of  different 
lengths  within  the  practical  limits  mentioned  above. 


VARIATION  OF  WAVE-LENGTHS  OF  CLOSED 
OSCILLATORY  CIRCUITS 

300.  The  wave-length  of  a  closed  oscillatory  circuit 

is  varied  in  exactly  the  same  way  as  that  of  an  open 

_       oscillatory  circuit,  namely,  by  in- 

_L    creasing  or  decreasing  the  capacity 

I  ^  g<    and  inductance  of  the  circuit. 

TS  301.  The  form  of  the  circuit, 
_  |  however,  lends  itself  more  easily 
_  to  increasing  the  wave-length  by 
1  <L  increasing  the  capacity  of  the 

-^—  -L.     g,         §5   circuit,  as  the  capacity,  instead  of 
__J  ^p    being  distributed  along  the  whole 

I       _  j      circuit  as  in  the  case  of  an  aerial, 
,  -  I      is  almost  entirely  concentrated  in 

c-^3   the  condenser. 

~3          C  302.  if  a  capacity  is  connected 

^3   in  parallel  with  another  capacity, 

L  -  WW^  -  '      the  total  capacity  is  increased  ;  thus 

by  connecting  an  additional  con- 

denser in  parallel  with  the  exist- 

ing condenser  of  a  closed  oscillatory  circuit  we  increase 

the  wave-length  of  that  circuit. 

303.  Fig.  43  shows  different  methods  of  increasing 
the  wave-length  of  a  closed  oscillatory  circuit. 

A  represents  the  original  circuit.     B  represents  the 
same  circuit  with  an  additional  capacity'  connected  in 


WIRELESS  TELEGRAPHY 


83 


parallel,  thus  increasing  the  total  capacity  and  thereby 
increasing  the  wave-length.  C' represents  the  same 
circuit  with  an  additional  inductance  connected  in  series, 
thus  increasing  the  total  inductance  of  the  circuit  and 
thereby  increasing  the  wave-length. 

304.  Fig.   44  represents  different  methods  used  for 
reducing  the  wave-length  of  a  closed  oscillatory  circuit. 


1 

T 


T 
T 


X 
T 


.  1 

'   f1 

T 

0     r^ 

A  represents  the  original  oscillatory  circuit.  B 
represents  the  same  circuit  with  an  additional  condenser 
connected  in  series  with  it,  thus  reducing  the  total 
capacity  of  the  circuit  and  thereby  reducing  the  wave- 
length. C  represents  the  same  circuit  with  some  of  the 
inductance  cut  out,  thus  reducing  the  total  inductance 
in  the  circuit  and  thereby  reducing  the  wave-length. 

305.  The  methods  above  described  are  only  used 
where  a  definite  jump  from  one  definite  wave-length  to 
another  is  required.  If  intermediate  wave-lengths  are 


84 


WIRELESS  TELEGRAPHY 


required,  it  is  usual  to  make  either  the  condenser  or  the 
inductance  adjustable,  as  shown  in  Fig.  45,  where  A 
represents  a  circuit  in  which  only  the  capacity  is  adjust- 
able, and  B  represents  a  circuit  in  which  only  the 
inductance  is  adjustable  ;  C  represents  a.  circuit  in  which 
both  the  capacity  and  the  inductance  are  adjustable. 


PRODUCTION  OF  OSCILLATING  CURRENTS 
IN  AN  AERIAL 

306.  The  methods  employed  for  causing  an  aerial  to 


FIG.  47. 

oscillate,  and  thus  radiate  electric  waves,  fall  under  two 
headings,  viz.  Direct  Excitation  and  Indirect  Excitation. 

DIRECT  EXCITATION  OP  THE  AERIAL 

307.  We  have  already  explained  that  an  aerial  con- 
nected to  earth  is  an  oscillatory  circuit,  and  therefore, 
for  convenience  in  explanation,  we  may  consider  it  as  a 
condenser  with  its  two  plates  connected  by  an  inductance, 
as  shown  in  Fig.  46,  the  aerial  wires  A,  Fig.  47,  corre- 
sponding to  one  plate  of  the  condenser  A,  Fig.  46 ;  the 


WIRELESS  TELEGRAPHY  85 

earth  E,  Fig.  47,  corresponding  to  the  other  plate  of  the 
condenser  E,  Fig.  4G  ;  and  the  connecting  wire  or  "  down- 
lead "  D,  Fig.  47,  corresponding  to  the  inductance  D, 
Fig.  46. 

308.  It  has  already  been  explained  (paragraph  237) 
that  if  a  condenser   be  charged  up  and  then  short- 
circuited  through  an  inductance,  the  charge  of  electricity 
will  not  immediately  come  to  rest,  but  the  condenser  will 
over-discharge  itself,  and  the  current  will  oscillate  back- 
wards and  forwards  until,  owing  to  the 
resistance  of  the  circuit  and  the  radiation 

of  energy,  the  charge  of  electricity  comes 
to  rest. 

309.  In  order  to  excite  an  oscillatory  cir- 
cuit, such  as  is  shown  in  Fig.  46,  it  is  there-         KK.  45 
fore  only  necessary  to  give  the  condenser 

an  initial  charge  of  electricity,  by  applying  a  "  voltage  " 
or  pressure  of  electricity  across  it,  and  then  allow  it  to 
discharge  itself  through  an  inductance. 

Let  us  now  see  how  this  can  best  be  accomplished. 

310.  We  can  charge  up  a  condenser  by  connecting  a 
battery  across  it,  as  shown  in  Fig.  48,  which  will  charge 
the  condenser  up  to  the  same  voltage  as  the  battery  ; 
but  in  applying  a  voltage  in  this  way  to  a  condenser, 
whose  two  plates  are  connected  together  through  an 
inductance  to  form  an  oscillatory  circuit,  as  shown  in 
Fig.   46,   the   electricity,   instead   of   charging   up   the 
condenser   as   desired,    will   simply    flow    through    the 
inductive  winding  D. 

311.  It  is  therefore  obvious  that  during  the  time  the 
condenser  is  being  charged,  we  must  break  the  circuit 
through  the  inductive  winding,  as  shown  in  Fig.  49  at 
the  point  marked  S. 


86  WIRELESS  TELEGRAPHY" 

312.  This,  however,  destroys  the  oscillatory  circuit, 
as  it  prevents  the  discharge  of  the  condenser  through 
the  circuit  D,  which  discharge  is  required  to  produce 
the  oscillations. 

313.  In  order,  therefore,  to  get  the  conditions  right, 
both  for  charging  up  the  condenser  and  for  discharging 
it  through  the  circuit  D,  it  would  be  necessary  to  devise 
some  form  of  mechanism  for  automatically  breaking 
the  discharge  circuit  and  connecting  the  battery  to  the 
condenser  at  one  moment,  and  then  "  making "  the 
discharge  circuit  at  the  next  moment. 

314.  This   method,   however,   is    impracticable,   as, 

apart  from  the  fact  that  it  would 
be  somewhat  complicated  in  oper- 
ation, an  additional  drawback 
arises  inasmuch  as  a  very  large 
battery  would  be  necessary  in 
FlG  49  order  to  charge  the  condenser  up 

to  a  sufficiently  high  voltage  to 

store  up  the  energy  that  is  required  to  obtain  a  useful 

range  of  transmission. 

315.  In  paragraph  286  we  showed  that  the  power 
put  into  a  condenser  depends  upon  three  things,  namely, 
the  capacity  of  the  condenser,   voltage  to  which  it 
"is  charged,  and  the  number  of  times  per  second  that  it 
is   charged.     Obviously,   therefore,   the  power  in  the 
oscillatory  circuit  is  also  equal  to  £  CV2lxS,  but  in  this 
case  S  will  be  the  number  of  times  per  second  that  the 
condenser  is  discharged  into  the  oscillatory  circuit. 

316.  As    already    explained    (paragraph    291),    the 
capacity  of  the  aerial  is  limited  by  the  wave-length  it  is 
desired  to  produce. 

Further,  the  number  of  times  per  second  it  can 


WIRELESS  TSLEOBAPHY  8fi 

be  charged  and  discharged  is  limited  by  other  practical 
considerations,  which  will  be  dealt  with  later. 

317.  It  follows  that  the  only  method  we  have  of 
increasing  the  power  in  the  oscillatory  circuit  we  are 
considering,   namely,   an   aerial,   is   by  increasing  the 
voltage  applied  to  the  condenser. 

Let  us  take,  for  example,  a  small  "  umbrella  "  aerial 
supported  by  a  mast  30  feet  high,  the  length  of  the 
radial  wires  forming  the  aerial  being  70  feet  long,  as 
shown  in  Fig.  47.  The  capacity  of  such  an  aerial 
would  be  about  '0005  microfarad. 

318.  Assuming  that  our  automatic  device  for  charging 
and  discharging  the  aerial  is  capable  of  doing  it  at  the 
rate  of  100  times  per  second,  it  can  be  shown  that  the 
initial  voltage  to  which  such  an  aerial  would  have  to 
be  charged  in  order  to  radiate  10  watts  of  power  would 
be  about  20,000  volts,  assuming  that  all  of  the  power  is 
expended  in  radiation  and  none  lost  in  the  resistance 
of  the  aerial  circuit. 

The  impracticability  of  the  method  described  above 
becomes  obvious,  as  it  would  require  a  battery  of 
about  14,000  dry  cells,  or  10,000  accumulator  cells,  to 
obtain  this  voltage. 

319.  A  very  much  simpler  method  of  exciting  an 
oscillatory  circuit  presents  itself  by  making  use  of  the 
properties  of   a    spark    gap    in    conjunction  with   an 
induction  coil  (described  in  paragraph  118  onwards) 

320.  Air  in  its  normal  state  is  nearly  a  perfect  in- 
sulator ;  that  is  to  say,  for  all  practical  purposes  it  will 
not  conduct  electricity.    If,  however,  a  sufficiently  high 
voltage  is  applied  across  an  air  space  the  insulation  of 
the  air  is  broken  down,  allowing  the  current  to  pass 
through  the  air  space,  causing  a  spark  to  occur,  and  the 


88  WIRELESS  TELEGRAPHY 

effect  is  to  make  the  air  space  momentarily  into  a 
conductor. 

Further,  once  the  spark  is  formed  it  will  be 
maintained  by  a  very  small  current,  but  as  soon  as  the 
succession  of  sparks  ceases  the  air  space  returns  to  its 
normal  state  of  insulation. 

321.  By  applying  this  phenomenon:  to  the  oscillatory 
circuit,  as  shown  in  Fig.  50,  we  get  conditions  such  that 
daring  the  time  that  the  condenser  is  being  charged  the 
path  through  the  inductive  winding  is  broken  by  the 
air-gap,  but  as  soon  as  the  voltage  across  the  condenser 
rises  to  a  certain  maximum,  depending  upon  the  length  of 
the  air-gap,  the  insulation  of  the  air-gap  is  broken  down, 
a  spark  occurs  across  it,  and  for  the  moment  the  gap, 
instead  of  being  an  insulator,  becomes  a  conductor,  and 
allows  the  condenser  to  discharge  itself  through  the 
oscillatory  circuit. 

322.  As  already  explained,  the  condenser  not  only 
discharges  itself,   but  over-discharges   itself,   and  the 
current  oscillates  backwards  and  forwards  a  number  of 
times,  until,  owing  to  the  resistance  of  the  circuit  and 
the  radiation  of  the  energy  in  the  form  of  waves,  the 
oscillations  die  down,  and  the  current  flowing  is  not 
sufficient  to  maintain  the  spark.    The  spark  then  goes  out 
and  the  air-gap  assumes  its  normal  insulating  properties 
until  the  next  high-voltage  impulse  is  applied  to  the  con- 
denser, when  the  same  cycle  of  events  takes  place. 

323.  Such  an  arrangement  is  shown  diagrammatic- 
ally  in  Fig.  50,  where  A  is  the  impulsive  high-voltage 
generator,  B  is  the  condenser,  C  is  the  inductance,  and 
D  the  spark-gap. 

324.  This  method  of  excitation  can  be  applied  to  a 
closed  oscillatory  circuit,  as  already  shown,  or  it  can  be 


WIRELESS  TELEGRAPHY  89 

applied  to  an  aerial  by  connecting  the  spark-gap  between 
the  aerial  and  earth,  as  shown  in  Fig.  51. 

325.  An   aerial  directly  excited    in  this  manner  is 
Dually  called  "  plain  aerial,"  and  is  extremely  efficient 


FIG.  50. 


FIG.  51. 


for  obtaining  a  comparatively  long  range  with  the  use 
of  a  small  power. 


COUPLED    OSCILLATORY    CIRCUITS 

326.  We  have  seen  how  an  oscillatory  circuit  can  be 
energised  by  charging  up  a  condenser  to.  a  high  voltage 
by  means  of  an  induction  coil  and  allowing  it  to  discharge 
through  an  inductance  and  air-gap.     Referring  to  Fig.  50, 
we  see  that  the  right-hand  part  of  the  diagram  is  drawn 
in  thick  lines.     This  is  a  convenient  way  of  denoting 
the  oscillatory  portion  of  the  circuit,  and  as — especially 
in  complicated  diagrams — it  is  of  great  importance  to 
distinguish   between   the   oscillatory   circuits   and   the 
"  low-frequency "  circuits,  such  as  the  induction  coil 
windings  and  leads,  the  reader  is  advised  to  follow  this 
plan  throughout. 

327.  Looking  at  Fig.  50  it  may  be  asked,  how  can  the 


90  WIRELESS  TELEGRAPHY 

thick  lines  be  said  to  form  a  "  circuit "  at  all  since  there 
is  a  distinct  gap  D  ?  A  little  thought  will  show  us  that 
the  gap  is  only  a  break  in  the  circuit  while  the  condenser 
is  being  charged  up  by  the  coil,  during  which  time  there 
are  no  oscillations  and  the  circuit  is  not  oscillatory ; 
but  when  the  voltage  of  the  condenser  has  reached  the 
value  necessary  to  break  down  the  insulation  of  the  air 
between  the  spark  balls  a  spark  takes  place,  and  the 
gap  then  becomes  a  conductor,  and  the  circuit  is  truly 
a  closed  one  ;  it  is  during  this  time  only  that  the  oscilla- 
tions take  place,  so  that  the  circuit  is  completed  and 
forms  a  "  closed  oscillatory  circuit."  Similarly,  in  the 
next  diagram,  Fig.  51,  when  the  spark  takes  place  the 
circuit  is  completed  and  forms  an  "  open  oscillatory 
circuit." 

FACTORS  LIMITING  THE  POWER  IN  OSCILLATORY 
CIRCUITS 

328.  Speaking  in  general,  we  may  say  that  the  ultimate 
factor  limiting  the  power  we  are  able  to  supply  to  any 
oscillatory  circuit  by  any  given   method  is  the  wave- 
length to  which  the  oscillatory  circuit  must  be  adjusted. 

329.  We  have  seen  that  the  power  which  can  be  put 
into  any  oscillatory  circuit  depends  upon  three  factors, 
namely : 

(1)  The  number  of  times  per  second  that  the  condenser 
is  charged ; 

(2)  The  voltage  to  which  the  condenser  is  charged  ; 
and 

(3)  The  total  capacity  of  the  condenser. 

We  find  that  all  these  factors  are  limited  by  practical 
considerations. 

330.  Taking  the  first  factor,  namely,  the  number  of 


WIRELESS  TELEGRAPHY  91 

times  per  second  that  the  condenser  is  charged,  we  shall 
show  later  that  the  note  produced  in  the  telephones  of 
the  receiving  station  is  the  same  frequency  as  the  frequency 
of  the  spark  of  the  transmitting  station. 

331.  This  is  called  the  spark  frequency,  and  must  not 
be  confused  with  the  oscillation  frequency  (vide  para- 
graph 257),  of  which  it  is  absolutely  independent. 

332.  The    human    ear    cannot    hear   a    note    whose 
frequency  exceeds  a  certain  value,   about  15,000  per 
second,  but  before  this  limit  is  reached  another  practical 
difficulty  arises,  namely,  the  mechanical  construction  of 
a  generator  to  produce  such  a  high  frequency. 

333.  Taking  the  second  factor,  namely,  the  voltage  to 
which  we  charge  a  condenser,  we  find  we  are  limited 
here  in  several  directions. 

334.  In  the  first  place,  if  the  oscillatory  circuit  we  are 
energising  is  an  aerial,  the  mere  fact  of  charging  it  to  a 
very  high  potential  is  in  itself  bad,  for  at  a  certain  voltage 
the  ends  of  the  wires  begin  to  "  brush  "  and  discharge 
electricity  to  the  surrounding  air,  causing  a  considerable 
loss  of  energy.     This  phenomenon  can  sometimes  be  seen 
at  night  at  the  ends  of  an  aerial  wire,  which  appear  to 
be  surrounded  by  a  bluish  glow. 

335.  Secondly,  the  difficulty  of  maintaining  a  suf- 
ficiently good  insulation  of  the  aerial  to  withstand  such 
a  high  voltage  becomes  very  serious,  especially  in  wet 
weather. 

336.  Thirdly,    to    charge   up    the   condenser   in   an 
oscillatory  circuit,  to  a  high  voltage  necessitates  using  a 
long  air-gap  in  the  oscillatoiy  portion  of  that  circuit  so 
that  it  may  not  break  down  until  a  high  voltage  is 
reached.     Although,  as  we  have  said,  the  air  becomes 
momentarily  a  conductor  when  the  spark  is  passing,  yet, 


92  WIRELESS  TELEGRAPHY 

like  all  conductors,  it  has  a  certain  resistance,  and  this 
resistance  increases  very  rapidly  with  the  length  of  the 
gap. 

337.  As  we  have  already  shown,  the  introduction  of 
resistance  in  an  oscillatory  circuit  causes  a  waste  of 
energy  and  a  rapid  dying  away  of  the  oscillations.    For 
this  reason  the  use  of  extremely  high  voltages  in  oscilla- 
tory circuits,  necessitating,  as  it  does,  a  long  spark-gap, 
leads  to  inefficiency. 

338.  Taking  the  third  factor,  namely,  the  total  capacity 
of  the  condenser,  we  find  that  an  increase  in  the  capacity 
in  any  oscillatory  circuit  will  necessarily  increase  the 
length  of  the  wave,  unless  a  corresponding  decrease 
is  made  in  the  inductance  of  the  circuit  (vide  para- 
graph 256). 

339.  We  find,  however,  that  in  a  closed  oscillatory 
circuit  we   can  reduce  the  inductance  (and  therefore 
increase  the  capacity)  to  a  far  greater  extent  than  we 
can  in  an  aerial. 

340.  In  the  case  of  an  aerial  we  can  increase  the 
capacity  by  bringing  the  aerial  nearer  the  ground,  and 
thus  reducing  the  thickness  of  the  dielectric  (vide  para- 
graph 19),  but  this  decreases  the  range  we  can  obtain. 

341.  We  can  also  increase  the  capacity  of  an  aerial 
by  increasing  the  length  of  the  wires  forming  the  aerial, 
but  this  at  the  same  time  increases  the  inductance  in 
the  wires,  and  therefore  increases  the  wave-length. 

342.  The  only  other  way  of  increasing  the  capacity 
of  an  aerial  is  to  increase  the  num'oer  of  wires  forming  it. 
This,  however,  will  not  increase  the  capacity  sufficiently 
for  our  purpose,  and,  moreover,  tends  to  make  the  aerial 
costly  and  unwieldy. 

343.  As  already  pointed  out,  however,  in  the  case  of 


WIRELESS  TELEGRAPHY  93 

a  closed  oscillatory  circuit,  the  proportion  of  the  capacity 
to  the  inductance  of  the  circuit  for  a  given  wave-length 
can  be  made  far  greater  than  in  the  case  of  an  aerial,  and 
therefore  we  can  make  such  a  circuit  capable  of  utilising 
a  larger  amoujit  of  power  for  the  same  wave-length,  the 
same  spark  frequency,  and  the  same  voltage. 

344.  A  closed  oscillatory  circuit,  however,  is  not  a 
good  radiator  (vide  paragraph  289),  and  is  therefore  not  a 
good  substitute  in  this  respect  for  the  open  oscillatory 
circuit  provided  by  the  aerial.     If,  however,   we  can 
combine  the  good  energy-storing  property  of  the  closed 
oscillatory  circuit  and  the  good  energy- radiating  property 
of    the  aerial,   we  shall  obtain  the   best  results  for  a 
limited  wave-length. 

345.  This  is  the  plan  on  which  is  based  the  "  coupled- 
circuit  "  transmitter  now  in  general  use,  a  diagram  of 
which  is  shown  in  Fig.  52. 

346.  The   closed  oscillatory 
circuit  X  (Fig.  52)  is  excited  in 
the  way  described  in  paragraph 
319.     The  oscillating  currents 
set  up  pass  to  and  fro  round  the 
circuit,  which  includes  a  coil 
L,  consisting  of  one  or  more 
turns  of  wire.     This  coil  is  so 
placed  with  respect  to  another 
coil  N  connected  in  the  aerial 
circuit  that  the  two  coils  exer- 
cise mutual  induction  (see  paragraph  114)  on  each  other. 
One  end  of  this  second  coil  is  connected  to  the  aerial  A, 
and  the  other  end  to  earth  E.     The  oscillating  currents 
flowing  through  L  create,  through  the  mutual  inductance 
of  the  two  coils,  oscillating  currents  in  the  aerial. 


94  WIRELESS  TELEGRAPHY 

347.  If,  however,  the  values  of  capacity  and  induct- 
ance of  the  aerial  circuit  are  arranged  so  that  the  aerial 
has  a  frequency  different  .from  that  of  the  closed  circuit, 
the  aerial  will  try  to  oscillate  at  its  own  frequency  (vide 
paragraph  252)  in  opposition  to  the  oscillations  put  into 
it  by  the  closed  circuit,  with  the  result  that  one  set  of 
oscillations  will  interfere  with  the  other,  and  very  little 
energy  will  be  transferred  from  the  closed  to  the  open 
circuit.    Under  these  conditions  the  two  circuits  are  said 
to  be  "  out  of  tune." 

348.  To  understand  what  happens  when  the  circuits 
are  out  of  tune,  we  may  take  it  that  the  first  oscillation  in 
the  closed  circuit  induces  a  wave  in  the  aerial  coil ;  this 
wave  travels  up  the  aerial,  reaches  the  free  insulated 
end,    turns  back  and.  tries  to  return   to   earth ;   but 
on  its  way  there  it  meets  another  wave  coming  up 
the  aerial,  induced  by  the  second   oscillation  in  the 
closed  circuit,  which  is  not  "  keeping  time  "  properly 
with  the  aerial,  and  these  two  waves  partly  destroy 
one  another. 

349.  But  if  the  aerial  circuit  is  so  arranged  as  to  have 
the  same  frequency  as  the  closed  circuit,  the  first  wave, 
instead  of  meeting  a  contrary  wave,  will  travel  down  to 
earth  unhindered,  and  as  it  swings  back  again  it  will 
find  the  second  wave,  induced  from  the  closed  circuit, 
ready  to  join  it  in  its  progress  up  the  aerial  and  down 
again  to  earth  ;  and  this  will  go  on,  one  wave  adding  on 
to  the  others  already  in  the  aerial,  until  the  condenser  C 
is  discharged  ;  that  is  to  say,  until  the  energy  originally 
stored  up  in  the  closed  oscillatory  circuit  is  transferred  to 
the  aerial.    Under  these  conditions  the  two  circuits  are 
said  to  be  "  in  tune." 

350.  We   may   say,  then,  that  in   order   to  excite 


WIRELESS  TELEGRAPHY 


95 


efficiently  one  oscillatory  circuit  from  another  in  which 
oscillating  currents  are  flowing,  it  is  necessary  that  the 
two  circuits  have  the  same  frequency. 

351.  A  simple  experiment  can  be  made  with  pendu- 
lums  to   illustrate  this  point.      A  piece  of  string  is 
stretched  between  two  fixed  points  (Fig.  53),  and  two 
pendulums,  P!  and  P2,  are  hung  from  it  a  short  distance 
apart. 

352.  Now  if  these  pendulums  have  the  same  time  of 


P: 


FIG.  53. 


swing,  and  therefore  the  same  frequency,  they  may  be 
said  to  be  in  tune,  and  it  will  be  found  that  if  Px  (which 
may  be  taken  to  represent  the  closed  oscillatory  circuit) 
be  started  swinging,  it  will,  owing  to  its  being  coupled 
to  P2  by  the  string,  gradually  start  a  similar  swing 
in  P2. 

353.  The  swing  in  P2  will  get  greater  and  greater  until 
the  energy  that  was  originally  put  into  Pl  is  transferred 
to  P2,  and  Pj  will  have  come  to  rest. 

354.  If,  however,  P2  be  made  shorter  or  longer  than 


•96  WIRELESS  TELEGRAPHY 

Pj,  so  as  to  have  a  different  frequency,  the  two  pendu- 
lums may  be  said  to  be  out  of  tune,  and  it  will  be 
found  that,  although  a  certain  amount  of  swing  will  be 
induced  in  P2,  the  two  pendulums  will  interfere  with 
one  another,  and  both  will  come  to  rest  after  erratically 
jerking  about. 

355.  The  closed  oscillatory  circuit  is  spoken  of  as  the 


FIG.  54. 

"  primary "  circuit ;  the  two  coils  L  and  N  form 
together  an  "  oscillation  transformer "  or  "  jigger," 
the  coil  L  being  the  "  jigger-primary,"  and  the  coil  N 
the  "  jigger-secondary." 

356.  In  order  to  "  tune  "  the  primary  circuit  to  the 
aerial,  it  is  usual  to  connect  a  "variable  inductance" 
both  in  the  aerial  circuit  and  in  the  primary  circuit,  as 
shown  in  Fig.  54.  Such  an  inductance  in  the  primary 
circuit  is  called  the  "  primary  tuning  inductance,"  and 
in  the  aerial  circuit  is  called  an  "  aerial  tuning  induct- 
ance." 


WIRELESS  TELEGRAPHY 


97 


THE  AUTO-JIGGER 

357.  In  the  above  method  of  indirect  excitation  we 
had  two  entirely  separate  circuits,  the  primary  circuit 
and  the  aerial  circuit,  connected  only  by  the  mutual 
induction  of  jigger-primary  and  jigger-secondary ;    and 
we  saw  that,  provided  each  of  these  two  circuits  was 
tuned  to  the  same  wave-length,  the  arrangement  offered 
us  an  excellent  combination — a  good  storer  of  energy 
combined  with  a  good  radiator  of  energy. 

358.  There  is  another  form  of  indirect  excitation, 
using  what  is  called  an  "  Auto-jigger,"  which  at  one  time 
was  fairly  extensively  used,  and  is  still  popular  among 
amateurs  owing  to  its  simplicity. 

359.  In  an  auto-jigger  we  still  have  the  two  circuits — 
the  primary  circuit  with  its 

condenser  and  j  igger-primary , 
and  the  aerial  circuit  with 
its  aerial,  its  tuning  induct- 
ance, its  jigger -secondary, 
and  its  earth,  and  these  must 
be  tuned  to  the  same  wave- 
length just  as  in  the  case  of 
the  ordinary  jigger,  but  in  the 
case  of  the  auto- jigger  the 
primary  circuit  is  in  actual 
metallic  connection  with  the 
aerial  circuit ;  in  fact,  the 
jigger-primary  is  formed  of  a 
certain  number  of  turns  of 
the  jigger-secondary  itself. 

360.  Thus  in   Fig.   55,  which  illustrates   the    auto- 
jigger,  the  aerial  circuit  consists  of  the  aerial  A,  the 


98  WIRELESS  TELEGRAPHY 

aerial  tuning  inductance  B,  the  jigger-secondary  CD,  and 
the  earth  connection  E;  while  the  primary  circuit  consists 
of  the  condenser  F,  the  spark-gap  G,  and  the  jigger- 
primary  D,  which  is  merely  a  certain  number  of  turns 
of  the  jigger-secondary  CD. 

361.  With  such  an  arrangement  we  have  the  same 
advantage  as  with  the  ordinary  jigger — namely,  a  good 
storer  of  energy  (the  closed  oscillatory  circuit  containing 
the  large  condenser  F)  transferring  its  energy,  through 
the  action  of  the  coil  D,  to  the  good  radiator,  the  open 
oscillatory  circuit  A,  B,  C,  D,  E, 

REACTION  OF  SECONDARY  ON  PRIMARY 
"362.  The  behaviour  of  a  coupled-circuit  transmitter, 
whether  an  ordinary  jigger  or  an  auto- jigger,  is  less 
simple  than  would  appear  at  first  sight. 

363.  It  might  seem  natural  to  suppose  that  since  the 
primary  circuit  has  the  same  wave-length  as  the  second- 
ary or  aerial  circuit,  it  would  simply  transfer  its  energy 
(put  into  it  by  the  induction  coil)  to  the  aerial  in  the 
form  of  oscillations  of  the  same  frequency  ;    and  that 
the  aerial  would  radiate  out  this  energy  in  the  form  of 
aether  waves  of  a  length  corresponding  to  that  frequency. 

364.  But  we  must  remember  that  just  as  the  currents 
flowing  to  and  fro  in  the  jigger-primary  induce  currents 
in  the  otherwise  passive  secondary,  so  do  the  currents 
thus  made  to  flow  in  the  secondary  act  on  the  primary 
and  induce  currents  in  it ;    so  that  when  the  primary 
circuit  has  given  up  its  energy  to  the  secondary  the 
latter  starts  giving  back  some  of  its  energy  to  the  prim- 
ary, which  returns  it  to  the  secondary,  and  so  on. 

365.  This  goes  on  till  so  much  energy  has  been  re- 
moved from  the  circuits — by  losses  in  resistance  and  by 


WIRELESS  TELEGRAPHY  99 

radiation  from  the' aerial — that  the  current  in  the  primary 
has  no  longer  power  to  cross  the  spark-gap,  when  the 
process  stops  until  it  is  started  again  by  the  induction 
coil  charging  up  the  condenser  once  more. 

3CG.  This  will  be  more  easily  understood  by  referring 
again  to  the  pendulum  experiment  described  in  para- 
graph 351. 

367.  In  this  paragraph  we  only  followed  the  action 
of  the  pendulums  up  to  the  moment  when  the  driving 
pendulum  Pl  had  transferred  its  energy  to  P2,  but  if 
we  watch  their  action  still' further  we  see  that  P2  now 
becomes   the    driving   pendulum,    and   its   energy   will 
gradually  be  transferred  back  to  Plf  and  this  transfer 
of  energy  goes  on  backwards  and  forwards  until  so  much 
energy  has  been  lost  in  friction  in  the  air  and  string  that 
both  pendulums  come  to  rest. 

Now  this  rather  complicated  give-and-take  process 
has  a  peculiar  effect  on  the  wave  set  up  in  the  aerial. 

368.  The  result  of  coupling  a  closed  oscillatory  circuit 
to  an  open  oscillatory  circuit,  each  of  which  is  tuned  to 
the  same  wave-length,  is  the  production  of  two  wave- 
lengths, one  longer  and  the  other  shorter  than  the  wave- 
length to  which  both  circuits  have  been  tuned.     These 
two  wave-lengths  are  known  as  the  Resultant  Wave- 
lengths. 

369.  It  is  not  an  easy  subject  to  understand,  but  it  is 
a  very  important  one,  and  our  readers  are  recommended 
to  take  pains  to  master  it. 

RESULTANT  WAVE-LENGTHS  OF  COUPLED  CIRCUITS 

370.  The   jigger-primary  has   a   certain   amount  of 
inductance  (which  has  already  been  defined)  due  entirely 
to  itself — its  number  of  turns,  its  diameter,  the  spacing 


100  WIRELESS  TELEGRAPHS 

of  its  turns,  etc. ;  this  is  called  the  self-inductance  of  the 
primary. 

371.  Similarly  the  jigger-secondary  has  a  certain  self- 
inductance,  due  to  its  number  of  turns,  diameter,  spacing 
of  turns,  etc. 

372.  But  besides  these  two  self-inductances,  which 
would  remain  unaltered  if  the  primary  were  taken  to  the 
Equator  and  the  secondary  kept  at  home,  there  is  a 
third  inductance  which  affects  both  primary  and  second- 
ary, and  which  is  due  to  the  proximity  of  the  one  coil  to 
bhe  other. 

373.  This  is  called  the  mutual  inductance;   thus  the 
primary   has,   in   addition  to   its  self-inductance,   the 
mutual  inductance  due  to  the  effect  of  the  secondary, 
and  the  secondary  has,  in  addition  to  its  self-inductance, 
the  mutual  inductance  due  to  the  presence  of  the  primary. 

374.  This  mutual  inductance  depends  on  the  position 
of  the  primary  with  regard  to  the  secondary,  on  their 
distance  apart,  and  on  the  number  of  turns  acting  on 
each  other. 

375.  The  mutual  inductance  of  two  such  coils,  though 
it  is  an  abstract  kind  of  thing  which  cannot  be  seen,  is 
nevertheless  a  definite  quantity,  and  is  very  important, 
as  it  is.  .through  the  agency  of  the  mutual  inductance 
that  the  primary  circuit  is  able  to  transfer  its  energy  to 
the  aerial  circuit. 

376.  By  making  a  simple  experiment  with  two  coils 
of  wire,  we  can  demonstrate  what  is  the  effect  of  the 
mutual  inductance  of  the  two  coils  on  the  total  self- 
inductance  of  the  coils. 

377.  Let  us  suppose  that  we  have  two  separate  coils 
A  and   B,  each  consisting  of  two  turns  as  shown  in 
Fig.  56,  placed  at  such  a  distance  apart  that  none. of  the 


WIRELESS  TELEGRAPHY 


101 


magnetic  lines  of  force  produced  by  a  current  flowing 
through  A  pass  through  the  coil  B,  and,  therefore,  also 
none  of  the  lines  of  force  produced  by  B  pass  through  A. 
If  now  we  connect  the  two  coils  in  series,  as  shown,  and 
pass  a  current  through  both  of  them,  a  certain  number  of 


lines  of  force  will  be  induced  by  the  coil  A,  and  an  equal 
number  will  also  be  induced  by  the  coil  B. 

378.  Now  the  inductance  of  a  coil  is  proportional  to 
the  number  of  lines  which  thread  it,  when  unit  current 
flows  through  that  coil,  multiplied  by  the  number  of  turns 
in  the  coil  (assuming  that  all  the  lines  of  force  produced  by 
the  coil  thread  all  the  turns) .  Thus,  if  we  assume  that  the 
coils  A  and  B  each  consist 
of  two  turns,  aud  that  when  f 
a  current  of  one  ampere  flows 
through  them  four  lines  of  ,' 
force  are  produced  by  each 
coil,  then  the  inductance  of 
each  coil  will  be  4x2  =  8, 
and  under  these  conditions 
the  total  inductance  of  the 
two  coils  together  will  be  8+8  =16. 

37(J.   Let  us  now  suppose  that  the  two  coils  be  placed 


102  WIRELESS  TELEGRAPHY 

rather  closer  to  one  another,  as  shown  in  Fig.  57,  so 
that  half  of  the  lines  produced  by  coil  A  also  thread 
the  coil  B,  and  also  half  of  the  lines  of  the  coil  B  thread 
the  coil  A.  Then  it  is  evident  that,  when  a  current  of 
one  ampere  flows  through  the  two  coils,  although  each 
coil  individually  produces  the  same  number  of  lines 
as  before,  the  total  number  of  lines  threading  each  coil 
will  be  increased  by  one  half  the  lines  in  the  other  coil. 

380.  We  have  assumed  that  each  coil  produces  4  lines 
of  force  ;  therefore,  in  this  case,  a  current  of  one  ampere 
will  have  iJie  effect  of  causing  6  lines  of  force  to  thread 
each  coil,  and  the  total  inductance  of  the  two  coils 
together  will  then  be  (6  x  2)  +  (6  x  2)  -12  + 12  -24. 

381.  Now  the  difference  between  this  total  induct- 
ance and  the  total  inductance  of  the  two  coils,  when 
a  great  distance  apart,  is  the   mutual  inductance  of 
the  two  coils.     That  is  to  say,  in  the  case  illustrated,  in 
Fig.  57,  mutual  inductance  =24  - 16  =8. 

382.  Similarly,  it  will  be  found  that  if  the  two  coils 
are  placed  so  close  together  that  all  the  lines  induced  by 
each  coil  individually  thread  the  other  coil,  the  total 
inductance    of    the   two   coils    together  will    then  be 
2  (turns)  x  8  (lines)  +  2  (turns)  x  8  (lines)  =16  +  16  =32, 
and  the  mutual  inductance  in  this  case  will  be  32  - 16  =16. 

383.  These  results  will  be  obtained,  provided  the 
direction  of  the  magnetic  lines  is  the  same  in  both  coils 
(vide  paragraph  82). 

384.  If,  however,  we  connect  up  the  two  coils  as 
shown  in  Fig.  58,  so  that  the  current  flowing  in  one  coil 
is  in  the  opposite  direction  to  that  flowing  through  the 
other  coil,  thus  inducing  magnetic  lines  in  one  coil  in 
the  opposite  direction  to  the  magnetic  lines  in  the  other, 
then  different  results  will  be  obtained  when  the  two 


WIRELESS  TELEGRAPHY 


103 


coils  are  brought  together,  because  the  magnetic  field 
produced  by  one  coil  will  tend  to  neutralise  the  magnetic 
field  produced  by  the  other  coil. 

385.  When  the  two  coils  are  remotely  separated,  as 


FIG.  58. 

shown  in  Fig.  58,  so  that  none  of  the  lines  of  force  in- 
duced by  A  pass  through  B,  and  vice  versa,  then  the  total 
inductance  of  the  two  coils  will  be  the  same  as  in  the 
case  illustrated  in  Fig.  50,  namely  16.  But  when  the 
two  coils  are  placed  closer  to  one  another,  as  shown  in 
Fig.  59,  so  that  two  lines  produced  by  each  coil  thread  the 
other,  then  these  two  lines  „- -^--,.,^. . , 

being  in  the  opposite  direction 
will  neutralise  two  of  the 
lines  induced  by  that  coil,  so 
that  in  this  case  a  current 
of  one  ampere  will  have  the 
effect  of  causing  only  two 
lines  of  force  to  thread  each 
coil,  and,  therefore,  the 
total  inductance  of  the  two  coils  together  will  then 
be  (2x2) +  (2x2)  8. 

386.  Now  the  dill'ercnce  between  this  total  inductance 
and  the  total  inductance  of  the  tv.o  coils  when  a  great 
distance  apart,  as  shown  in  Fig.  58,  that  is  to  say  8  - 16, 


FIG. 


104  WIRELESS  TELEGRAPHY 

will  again  represent  the  mutual  inductance  of  the  two 
coils,  which  it  will  be  seen  is  the  same  value,  but  opposite 
in  sign  to  the  value  of  the  mutual  inductance,  when  the 
two  magnetic  fields  were  in  the  same  direction.  In  that 
case  the  mutual  inductance,  as  we  showed,  was  +8. 
In  this  case  the  mutual  inductance  is  -  8. 

387.  Similarly,  it  will  be  found  that  if  the  two  coils 
are  placed  so.  close  together  that  all  the  lines  induced 
by  each  coil  individually  thread  the  other  coil,  the  total 
inductance  of  the  two  coils  together  will  be  0,  and  the 
mutual  inductance  in  this  case  will  therefore  be  - 16. 

388.  Returning  to  the  problem  of  coupled  oscillatory 
circuits,  we  can  now  follow  the  effect  of  the  mutual 
inductance  on  the  resultant  wave-length. 

Let  us  suppose,  for  the  sake  of  simplicity,  that  the 
self-inductance  of  the  primary  circuit  is  equal  to  that 
of  the  secondary  circuit ;  we  know  that  the  wave- 
lengths of  the  two  circuits  are  the  same,  but  as  a  rule  the 
inductance  of  the  primary  is  much  less  than  that  of  the 
secondary,  so  as  to  enable  the  primary  condenser  to  be 
of  much  larger  capacity  than  that  of  the  aerial ;  there  is 
no  reason,  however,  why  we  should  not,  for  the  sake  of 
argument,  make  the  two  capacities  equal,  and  therefore 
the  two  inductances  also  equal.  Let  each  of  these 
inductances  be  L,  and  let  the  mutual  inductance  between 
primary  and  secondary  be  M. 

389.  Now  owing  to  the  give-and-take  process  which 
we  described  above,  the  relative  directions  of  the  oscilla- 
tory currents  flowing  in  the  primary  and  secondary  coils 
are  continually  changing  with  this  result :  it  makes  the 
mutual  inductance  M  add  itself  to  the  self-inductance  L 
at  one  moment,  and  then,  a  fraction  of  a  second  later,  it 
makes  M  subtract  itself  from  L.    The  result  is  that  at  the 


WIRELESS  TELEGRAPHY  105 

first  moment  each  circuit  behaves  as  if  its  total  inductance 
were  L  +  M,  and  at  the  next  moment  as  if  it  were  L  -  M. 
But  these  moments  are  so  close  together — separated 
only  by  such  an  infinitely  small  fraction  of  a  second — 
that  what  happens  is  that  the  circuits  appear  to  possess 
these  two  values  of  inductance  at  the  same  time  ;  so  that 
they  behave  as  if,  instead  of  each  having  an  inductance 
L,  they  each  had  two  different  inductances,  L  +  M  and 
L-M. 

390.  But  if  a  circuit  has  two  inductances  and  one 
fixed  capacity,  it  is  clear  that  it  will  give  two  wave- 
lengths ;    and,  as  a  matter  of  fact,  the  result  of  the 
give-and-take  action  between  primary  and  secondary  is 
that  the  aerial  sends  out  two  waves,  one  longer  and  one 
shorter  than  the  wave  to  which  both  the  primary  and 
aerial  circuits  were  tuned. 

391.  It  is  clear  that. the  production  of  these  two 
waves  is  governed  by  the  size  of  M  compared  with  L ; 
if  we  make  M  very  small  compared  with  L  by  increasing 
the  distance  between  the  primary  and  secondary  of  the 
jigger,  L  +  M  will  only  be  very  slightly  larger  than  L  -  M, 
so  that  the  two  waves  will  be  so  nearly  equal  as  to  be 
indistinguishable. 

392.  So  if  we  move  the  jigger-primary  farther  and 
farther  away  from  the  jigger-secondary,  we  can  reduce 
M  and  make  the  two  waves  approach  nearer  and  nearer 
to  one  another,  till  finally  they  merge  into  one  wave- 
length which  will  be  of  the  same  value  as  that  of  the 
circuits  taken  by  themselves. 

393.  We  assumed,  for  the  sake  of  simplicity,  at  the 
beginning  of  paragraph  388,  that  the  inductance  of  the 
primary  was  equal  to  that  of  the  secondary.     If,  as  is 
usual,  these  inductances  arc  different,  the  same  thing 


106  WIRELESS  TELEGRAPHY 

holds  good,  except  that  the  simple  formula  of  L  +  M 
and  L  -  M  becomes  somewhat  more  complicated  and 
elaborate. 

394.  To  summarise  we  may  say  that : 

(1)  Two  oscillatory  circuits  can  be  coupled  together 
for  the  purpose  of  exciting  one  from  the  other. 

(2)  The  two  circuits  must  be  both  tuned  to  the  same 
wave-length. 

(3)  The  result  is  the  production  of  two  distinct  waves, 
one  longer  and  one  shorter  than  the  normal  wave-length 
of  either  circuit  taken  separately. 

(4)  The  closer  two  circuits  are  coupled  together  the 
greater  the  difference  between  the  two  resulting  wave- 
lengths. 

CALCULATION  OF  THE  DEGREE  or  COUPLING 

395.  For  convenience  the  degree  of  coupling  between 
two  oscillatory  circuits  is  expressed  as  a  percentage  of 
the  full  coupling. 

A,  large  degree  of  coupling  is  known  as  a  "  close 
coupling,"  and  a  small  degree  of  coupling  as  a  "  loose 
coupling." 

396.  If  two  oscillatory  circuits  were  fully  coupled 
the  two  resulting  waves  would  be  so  far  apart  that  the 
lower  wave  would  be  sensibly  zero,  and  the  only  wave- 
length left  would  be  \/2  or  1'4  times  the  wave-length 
of  the  two  circuits  taken  separately. 

397.  In  practice  such  conditions  cannot  be  obtained, 
because  even  if  the  primary  and  secondary  coils  were 
so.  close  together  that  all  the  lines  of  force  induced  by 
the  primary  coil  threaded  the  secondary  coil,   and  vice 
versa,  these  are  not  the  only  lines  of  force  induced   by 
each  circuit. 


WIRELESS  TELEGRAPHY  107 

398.  For  instance,  the  currents  flowing  through  the 
conductors  which  connect  the  primary  coil  to  the  con- 
denser  and   through  the   primary  tuning   inductance, 
induce  lines  of  force  round  those  conductors  which  do 
not  thread  any  part  of  the  secondary  circuit ;  similarly, 
the  currents  flowing  through  the  radiating  portion  of 
the  aerial,  as  well  as  through  the  aerial  tuning  inductance 
coil,  induce  lines  of  force  which  do  not  thread' any  part 
of  the  primary  circuit. 

399.  Since  the  degree  of  coupling  is  the  proportion  of 
the  number  of  lines  which  thread  the  other  circuit  to  the 
total  number  of,  lines  induced,  it  is  obvious  that  a  full 
coupling  between  the  primary  circuit  and  the  aerial  is 
impossible.     These  conditions  will  be  more  clearly  under- 
stood by  referring  to  Fig.  60,  which  shows  diagrammatic- 
ally  the  distribution  of  the  lines  of  force  induced  in  the 
two  circuits. 

400.  It  will  also  be  seen  that  the  greater  the  amount  of 
tuning  inductance  that  is  inserted  in  either  the  primary 
or  secondary  circuit,  the    less  will   be  the  degree  of 
coupling  between  the  two  circuits,  unless  these  induct- 
ances be  so  arranged  relatively  to  one  another  that  the 
lines  of  force  induced  by  the  currents  flowing  through 
them  thread  the  inductance  coils  of  the  other  circuit. 

401.  It  is  evident  that  any  "  outside"  inductance  (i.e. 
any  inductance  which  is  not  mutually  acting  on  the  other 
circuit)  included  in  either  the  Primary  or  the  Secondary 
circuits  is    tending  to    weaken  the  coupling  obtainable 
between  the  two  circuits. 

It  is  not,  however,  necessarily  an  advantage  to  have 
a  close  coupling  between  the  two  circuits,  and  in  fact 
too  close  a  coupling  has  many  disadvantages. 

402.  In  practice,  it  is  usual  to  allow  for  a  maximum 


108 


WIRELESS  TELEGRAPHY 


coupling  of  15  per  cent  or  20  per  cent  between  the 
primary  circuit  of  the  transmitter  and  the  aerial  circuit, 
and  where  very  sharp  teming  is  required  this  coupling 


7F^<;,,;"-'y 

'/!  \-_y 


Primary  Tuning 


^~x««S^ 


FIG.  60. 


is  frequently  reduced  to  1  per  cent  or  2  per  cent,  by 
methods  described  later. 

403.  As  a  matter  of  fact,  with  commercial  stations, 
a  regulation  has  been  laid  down  by  the  International 


WIRELESS  TELEGRAPHY  109 

Wireless  Convention  that  no  station  is  allowed  to  use  a 
closer  coupling  than  15  per  cent. 

404.  Since  the  difference  between  the  two  resulting 
wave-lengths  of  coupled  circuits  depends  upon  the  degree 
of  coupling  between  the  circuits,  it  follows  that  we  can 
calculate  the  coupling  if  we  know  the  values  of  these 
two   waves,  and  the  following  formula,  although  not 
exact,  will  give  a  very  near  approximation  of  the  per- 
centage of  coupling. 

405.  If    k  =  percentage    of    coupling    between    two 
circuits,  and  Xj  is  the  wave-length  of  the  longer  of  the 
two  resulting  waves,  X2  the  wave-length  of  the  shorter 
of  the  two  resulting  waves,  and  X0  the  wave-length  of 
each  of  the  circuits  taken  separately,  then 

k  =  — — ?  x  100  (approximately). 

^o 

406.  Let  us  apply  this  formula  to  a  practical  case. 
On  a  certain  vessel  a  wireless  installation  had  Jbeen 

fitted.  The  wave-length  of  the  closed  oscillatory  circuit 
was  adjusted  to  600  metres,  and  that  of  the  aerial  circuit 
to  the  same.  When  the  primary  oscillatory  circuit  was 
coupled  to  the  aerial  it  was  found  that  the  resulting 
wave-lengths  were  570  metres  and  630  metres  respec- 
tively. 

From  this  it  can   be  calculated  that  the  coupling 
between  the  two  circuits  was  10  per  cent,  for — 

»-«±5«xlOi 

600 

=  10  per  ceni 


110  WIRELESS  TELEGRAPHY 

METHODS  OF  VARYING  THE  COUPLING  BETWEEN 
TWO  OSCILLATORY  CIRCUITS 

407.  Many  different  ways  are  employed  for  varying 
the  coupling  between  the  inductive  windings  of  two  oscil- 
latory circuits ;  all  of  them,  however,  are  based  on  the 
principle  of  varying  the  proportion  between  the  number 
of  lines  induced  by  one  coil,  which  thread  the  other  coil, 
and  the  total  number  of  lines  induced. 

408.  The  method  most  commonly  used  to  vary  the 
coupling  between  the  primary  circuit  and  the  aerial 
circuit  of  a  transmitter  is  to  slide  the  secondary  winding 
away  from  the  primary  winding. 

This  method  is  illustrated  in  Figs.  61  and  62,  where 
A  is  the  inductive 
winding  of  the  open 
oscillating  circuit,  that 

B  secondary'  and  B  the 

FTC,  61.  Ffa.,6*  inductive   winding  ^  of 

the   closed   oscillating 
circuit,  that  is  to  say,  the  jigger-primary. 

409.  When  one  of  these  two  coils  is   immediately 
above  the  other,  as  shown  in  Fig.   61,  the  coupling 
between  the  two  is   at  its  maximum,  but  when  the 
secondary  winding  is  moved  until  it  occupies  a  position 
near  the  edge  of  the  primary  winding,  as  shown  in  Fig. 
(52,  the  coupling  is  at  its  minimum. 

410.  Another    method    of    adjusting    the    coupling 
between  two  circuits  is  to  alter  the  relative  angular 
position  between  the  axes  of  the  two  windings. 

When  these  two  axes  are  in  line   the  coupling  is 
at  its  maximum,  and  when  they  are  at  right  angles  to 


WIRELESS  TELEGRAPHY  111 

one  another  the  coupling  is  at  its  minimum.  In  this 
case  the  lines  of  force  induced  by  the  one  coil  do  not 
thread  the  other  coil,  but  pass  along  the  conductors. 

This  method  is  illustrated  in  Figs.  63  and  64.  Iu 
Fig.  63  the  axes  of  the  two 
coils  are  in  line,  and  the 
coupling  is  at  its  maximum, 
whereas  in  Fig.  64  the  axes 
of  the  two  coils  are  at 
right  angles  to  one  another, 
and  the  coupling  is  at  its 
minimum.  FIG.  63.  FIG.  64. 


THE   WAVEMETER 

411.  The  Wavemeter  is  an  instrument  for  measuring 
the  frequency  and  therefore  the  length  of  the  wave  or 
waves  emitted  by  oscillatory  circuits. 

412.  Briefly,  it  consists  of  a  closed  oscillatory  circuit 
whose  wave-length,  or  more  strictly  speaking,  frequency, 
it  is  possible  to  vary,  connected  to  a  detector,  by  means 
of  which  it  is  possible  to  tell  the  comparative  amount  of 
current  flowing  in  the  oscillatory  circuit. 

413.  To  measure  the  wave-length  of  an  oscillatory 
circuit,  the  instrument  is  brought  sufficiently  near  some 
part  of  that  circuit,  so  that  the  oscillating  currents 
flowing  in  it  will  induce  similar  currents  in  the  oscillatory 
circuit  of  the  Wavemeter.  ,   «v 

414.  In  paragraph  346  we  showed  that  when  two 
oscillatory  circuits  are  coupled  together,  one  of  which 
is  set  oscillating,  similar  oscillations  are  induced  in  the 
second  circuit,  provided  that  the  two  circuits  are  in  tune  ; 
that  if  they  are  out  of  tune,  although  a  certain  amount 


112  WIRELESS  TELEGRAPHY 

of  current  is  still  induced  in  the  second  circuit,  this 
current  will  be  comparatively  feeble  and  erratic,  but  will 
rapidly  rise  as  the  circuits  are  brought  nearer  and  nearer 
into  tune,  reaching  a  maximum  when  the  two  circuits 
are  quite  in  tune. 

415.  When  we  bring  the  wavemeter  near  another 
oscillatory  circuit,  we  are  in  effect  coupling  the  two 
circuits  together,  and  we  shall  obtain   similar  pheno- 
mena. 

416.  By  adjusting  the  frequency  of  the  wavemeter 
circuit,  and  at  the  same  time  noting,  by  means  of  the 
detector,  the  comparative  amount  of  current  induced 
into  it,  we  can  tell  exactly  when  the  wavemeter  circuit 
is  in  tune  with  the  circuit  we  are  measuring,  for  when  the 
circuits  are  in  tune  the  current  will  be  strongest.    If  we 
know  the  value- of  the  wave-length,  to  which  the  wave- 
meter  circuit  is  adjusted,  it  follows  that  this  wave-length 
is  also  the  wave-length  of  the  circuit  we  are  measuring. 

THE  OSCILLATORY  CIRCUIT  OP  A  WAVEMETER 

417.  In  practice  it  is  usual  to  vary  only  the  capacity 
of  the  circuit,  keeping  the  inductance  a  constant  value 
throughout.     This  for  various  practical  reasons  is  found 

to  be  more  convenient  than  ad- 
justing the  inductance.  Fig.  65 
shows  such  an  oscillatory  circuit, 
where  L  is  the  fixed  inductance 
and  C  the  variable  condenser. 

418.  We  have  already  learnt  that  the  wave-length 
of  an  oscillatory  circuit  depends  upon  the  product  of 
the  capacity  and  the  inductance  of  that  circuit :    it 
follows,  therefore,  that  such  a  circuit  can  be  "  tuned  up," 
or,  in  other  words,  adjusted  to  the  same  frequency  as 


WIRELESS  TELEGRAPHY  113 

that  of  the  circuit  whose  wave-length  it  is  required  to 
measure. 

419.  Practical    considerations    limit    the    maximum 
and  minimum  values  of  the  capacity  to  which  the  con- 
denser can  be  adjusted,  and  therefore  limit  the  maximum 
and  minimum  wave-lengths  to  which  the  circuit  can  be 
tuned. 

420.  An  illustration  of  a  variable  condenser  is  shown 
in  Fig.  66.     The  principle  on  which  it  is  constructed 
will  be  described  later,  but  for  the  present  it  is  sufficient 
to  know  that  its  capacity  is  varied 

by  turning  the  handle  A.  Fixed  to 
this  handle  is  a  pointer  B,  which 
passes  over  a  scale  C.  This  scale  is 
carried  half-way  round  the  circum- 
ference of  the  condenser,  and  is 
divided  into  a  number  of  equal  divi- 
sions which  are  marked  from  0  to 
100.  When  the  handle  of  the  condenser  is  so  turned 
that  the  pointer  indicates  the  figure  0,  the  capacity  of 
the  condenser  is  at  its  minimum,  and  as  the  pointer 
passes  up  the  scale  the  capacity  of  the.  condenser  increases 
until  it  arrives  at  its  maximum  capacity  when  the 
pointer  indicates  the  figure  100. 

421.  When  this  condenser  forms  part  of  an  oscillatory 
circuit,  the  inductance  of  which  is  fixed,  as  in  the  case 
of  the  wavemeter,  it  follows  that  the  wave-length  of  the 
circuit  will  have  a  definite  value  for  every  definite  position 
of  the  condenser  pointer 

422.  These  wave-lengths  are  carefully  and  accurately 
measured  after  the  instrument  is  made  (by  methods 
which  for  the  purpose  of  this  book  it  is  unnecessary  to 
explain),  and  a  list  or  chart  is  supplied  with  the  instru- 

I 


114  WIRELESS  TELEGRAPHY 

xnent  giving  the  wave-lengths  of  the  circuit  corresponding 
to  each  scale  reading  of  the  condenser. 

423.  By  the  use  of  this  chart  we  can  find  out  to  what 
wave-length  the  instrument  has  been  adjusted  by  refer- 
ring first  to  the  condenser  reading  and  then  reading  off 
the  chart  the  value  of  the  wave-length  corresponding 
to  that  condenser  reading. 

THE  "  DETECTOR  "  CIRCUIT  OF  A  WAVEMETER 

424.  In  order  to  tell  when  the  wavemeter  circuit  is 
in  tune  with  the  other  circuit,  we  must  find  a  means  of 
measuring  the  current  in  the  wavemeter  circuit  (vide 
paragraph  416). 

425.  It  is  not  necessary  to  know  the  actual  value  of  the 
current,  but  only  its  comparative  value,  so  that  a  detector 
Which  will  respond  proportionally  to  the  amount  of 
current  passing  through  it  will  suit  our  purpose. 

426.  The  telephone  receiver  is  a  very  suitable  instru- 
ment for  this  purpose  ;    for  one  thing,  it  is  extremely 
sensitive  to  even  the  smallest  current  passing  through  it, 
and  for  another  thing,  by  judging  the  loudness  of  the 
sound  in  the  telephone  we  can  judge  the  comparative 
amount  of  current  passing  through  it. 

427.  High-frequency  Oscillating  Currents,  however, 
will  not  affect  the  telephone  receiver,  as  the  alternations 
are  much  too  rapid  for  the  diaphragm  to  follow. 

428.  So  that,  to  enable  us  to  detect  the  high-frequency 
currents  produced  in  the  wavemeter,  these  currents, 
or  at  all  events  that  part  of  them  which  is  made  to  pass 
through  the  telephones,  must  be  rectified,  or,  in  other 
words,  converted  into  uni-dir$ctional  currents. 


WIRELESS  TELEGRAPHY  115 

THE  USE  OF  CRYSTALS 

429.  It  is  found  that  certain  crystals,  such  as  car- 
borundum, have  the  property  of  rectifying  high-frequency 
oscillating    currents.     They    really    act   as    non-return 
valves,  allowing  the  current  to  pass  through  them  in  one 
direction  only,  which  is  equivalent  to  converting  the  high- 
frequency  current  into  a  uni-directional  current. 

430.  These   crystals,    however,    have   an   extremely 
high  resistance,  and  for  this  reason  cannot  be  inserted 
directly   in   the  oscillatory   circuit.    A  little  thought, 
however,  will  show  us  that  it  is  not  necessary  to  insert 
either    the    crystal    or    the 

telephones  in  the  oscillatory 
circuit. 

431.  The   current  in   the 


oscillatory     circuit,     as     we  FlG  67 

know,   charges  up  the  con- 
denser of  that  circuit  to  a  certain  voltage ;  the  greater 
the  current  induced  in  the  inductance  coil  of  the  wave- 
meter  the  higher  the  voltage  to  which  the  condenser  will 
be  charged. 

432.  If,  therefore,  we  place  our  crystal  in  series  with 
the  telephone  across  the  condenser,  as  shown  in  Fig.  67, 
we  shall  not  in  any  way  interfere  with  the  oscillatory 
properties  of  the  oscillatory  circuit,  but  we  shall  get  a 
certain  current  passing  through  the  crystal  and  the 
telephones,  the  amount  of  which  will  depend,  upon 
the  voltage  to  which  the  condenser  is  charged,  and 
therefore  will  indicate  the  amount  of  current  induced 
in  the  oscillatory  circuit.  Moreover,  the  crystal  will 
rectify  this  current,  so  that  in  effect  we  shall  get  a 
uni-directional  current  passing  through  our  telephones. 


116  WIRELESS  TELEGRAPHY 

433.  The  current  also  will  be  an  intermittent  current, 
the  number  of  interruptions  per  second  being  the  same  as 
the  number  of  sparks  per  second  in  the  oscillatory  circuit 
which  is  being  measured  (vide  paragraphs  533  to  536). 
We  shall  therefore  get  a  buzz,  or  note,  in  the  telephone 
corresponding  exactly  to  that  produced  by  the  spark  of 
the  transmitter,  and  proportional  in  its  loudness  to  the 
amount  of  current  induced  in  the  oscillatory  circuit  of 
the  wavemeter. 

434.  It  is  clear,  therefore,  that  if  we  vary  the  adjust- 
able condenser  of  the  wavemeter  circuit,  and  at  the  same 
time  listen  to  the  sound  in  the  telephones,  when  this 
sound  is  loudest  the  wavemeter  circuit  is  in  tune  with 
the  oscillatory  circuit,  and  by  noting  the  position  of  the 
condenser  thus  obtained,  and  referring  this  reading  to 
our  chart,  we  find  the  value  of  the  corresponding  wave- 
length, and  therefore  the  length  of  the  wave  emitted 
by  the  oscillatory  circuit  being  measured. 

CONSTRUCTION  OF  AN  ADJUSTABLE  CONDENSER 

435.  The  construction  of  an  adjustable  condenser  is 
illustrated  in  Figs.  68,  69,  and  70.    A  number  of  semi- 
circular metal  plates  A  are  connected  together,  and  held 
rigidly  parallel  to  one  another  and  at  a  sufficient  distance 
apart  to  allow  the  second  set  of  metal  plates  B  to  pass  in 
between  them.     Fixed  to  the  upper  sides  of  both  the  A 
plates  and  the  B  plates  are  ebonite  plates  C  of  the  same 
shape.     The  second  set  of  metal  plates   B  are  held 
together  on  a  spindle  D,  which  can  be  rotated  by  the 
handle  E  which  is  fixed  to  one  end  of  the  spindle. 

The  fixed  plates  A  form  one  side  of  the  condenser, 
and  the  movable  plates  B  form  the  other  side  of  the 
condenser,  the  dielectric  of  the  ebonite  being  formed 


WIRELESS  TELEGRAPHY  117 


118  WIRELESS  TELEGRAPHY 

by  the  ebonite  plates  C.  When  these  plates  are  in  the 
positions  shown  in  Fig.  68,  the  capacity  of  the  condenser 
is  practically  zero,  but  if  the  movable  plates  B  are 
rotated  until  they  interleave  themselves  with  the  A 
plates  and  occupy  the  position  as  shown  in  Fig.  69,  the 
capacity  of  the  condenser  is  increased  to  half  its  maximum 
capacity,  as  half  the  surface  of  the  A  plates  is  acting 
through  the  dielectric  on  to  half  the  surface  of  the  B 
plates.  Finally,  if  the  B  plates  be  still  further  rotated 
until  they  occupy  positions  entirely  under  the  A  plates, 
as  shown  in  Fig.  70,  the  capacity  of  the  condenser  is  at 
its  maximum.  It  is  only  necessary  to  fix  to  the  moving 
plates  a  pointer  F,  which  will  pass  across  the  scale  G, 
and  thus  denote  the  exact  position  of  the  plates. 

WIRELESS  TELEGRAPH  RECEIVERS 

436.  A  wave  has  the  property  of  producing  a  dis- 
turbance similar  to  the  disturbance  which  started  the 
wave  (vide  paragraph  156). 

437.  We  have  shown  that  oscillating  currents  flowing 
in  an   open  oscillatory  circuit,  such  as  an  aerial,  will 
produce  electric  waves.    It  follows,  therefore,  from  the 
above,  that  electric  waves  will  produce  oscillating  currents 
in  an  aerial. 

438.  The  Receiver  is  that  part  of  the  apparatus  of  a 
Wireless  Telegraph  Station  which  converts  the  oscillating 
currents  produced  by  electric  waves  in  the  aerial  into 
visible  or  audible  signs. 

439.  The  frequency  of  the  oscillating  current  produced 
in  the  aerial  is  the  same  as  the  frequency  of  the  waves 
which  produce  it. 

440.  By  means  of  an  aerial  connected  to  a  receiver, 


WIRELESS  TELEGRAPHY  119 

therefore,  we  can  convert  the  electric  waves  which  are 
being  radiated  from  a  transmitting  station  into  visible 
or  audible  signs,  thus  enabling  us  to  "  read  "  the  message 
which  is  being  transmitted. 

ESSENTIALS  OF  A  RECEIVER 

441.  We  have  already  explained  that  an  aerial  forms 
an  "  open  "  oscillatory  circuit  and  has  a  natural  fre- 
quency of  its  own.    We  have  also  shown  that  an  oscil- 
lating current  will  not  flow  easily  in  a  circuit  unless  the 
frequency  of  that  circuit  is  the  same  as  that  of  the 
oscillating  current — that  is  to  say,  in  this  case  the  aerial 
circuit  must  be  in  tune  with  the  wave  which  is  to  be 
received. 

The  first  essential  of  a  receiver,  therefore,  is  a  variable 
inductance  and  a  variable  condenser,  which  can  be 
connected  in  series  with  the 
aerial  by  means  of  which 
the  latter  can  be  tuned  to  the 
desired  wave-length. 

442.  Fig.    71    illustrates 
these   connections,  where  A 
and   E   are    the    aerial    and 
earth  terminals  of  the  receiver, 

I  is  the  inductance — more  or  , 
less  of  which  can  be  included 
in  the  aerial  circuit  by  means  of  the  switch  Sj — and  C 
the  variable  condenser  across  which  is  fitted  a  short- 
circuiting  switch  S2. 

443.  The  inductance  I  is  called  the  "  Aerial  Tuning 
Inductance,"  and  the  condenser  C  the  "  Aerial  Tuning 
Condenser." 

444.  We  know  that  by  placing  a  condenser  in  series 


120  WIRELESS  TELEGRAPHY 

with  the  aerial  we  reduce  the  wave-length  of  the  aerial, 
and  by  placing  an  inductance  in  series  with  the  aerial 
we  increase  the  wave-length  of  the  aerial. 

445.  If,  therefore,  the  wave-length  which  it  is  required 
to  receive  is  shorter  than  the  natural  or  "  fundamental  " 
wave-length  of  the  aerial  we  must  cut  out  all  the  in- 
ductance in  the  circuit  by  means  of  the  switch  S1}  and  we 
must  reduce  the  value  of  the  adjustable  condenser  C 
until  the  correct  wave-length  is  obtained.     The  switch 
S2  will  in  this  case  be  open,  as  shown  in  the  diagram 
{Fig.  71). 

446.  If,  on  the  other  hand,  the  wave-length  which  it 
is  desired  to  receive  is  longer  than  the  fundamental  wave- 
length of  the  aerial,  in  order  to  bring  the  wave-length 
of  the  aerial  into  tune  we  must  first  short-circuit  the 
condenser  C  by  means  of  the  switch  S2,  thus  leaving 
no  capacity  in  series  with  the  aerial,  and  we  must  increase 
the  inductance  in  the  circuit  by  means  of  the  switch  Si 
until  the  correct  wave-length  is  obtained. 

METHODS  or  DETECTING  THE  OSCILLATING 
CURRENTS 

447.  The  next  essential  of  the  receiver  is  some  device 
whereby  the  presence  of  the  oscillating  currents  can  be 

'  detected. 

448.  In  paragraph  432  we  showed  how  this  could  be 
done,  in  the  case  of  a  wavemeter,  by  placing  across  the 
condenser  of  the  oscillatory  circuit  a  pair  of  telephones 
in  series  with  a  crystal.     The  telephones  in  series  with 
a  crystal   constitute  a  detector.    This  method  can  be 
adopted  in  the  receiver  by  placing  the  detector  across 
the  aerial  tuning  condenser,  but  it  is  not  an  efficient 
method  for  the  following  reason. 


,     WIRELESS  TELEGRAPHY  121 

449.  The  aerial  tuning  condenser  forms  only  a  part 
of  the  capacity  of  the  whole  aerial  circuit,  so  that 
although  the  detector  may  be  extremely  sensitive,  jt  is 
not  being  used  to  the  best  advantage. 

450.  Another  method  is  to  apply  the  detector  across 
tjie  aerial  tuning  inductance,  but  this  method  has*also 
the  same  disadvantage — viz.  that  we  are  only  applying 
the  detector  to  a  portion  of  the  whole  inductance  of 
the  aerial  circuit. 

451.  If,  however,  we  are  receiving  a  wave  very  much 
longer  than  the  natural  wave-length  of  the  aerial,  in 
order  to  tune  up  the  latter  we  naturally  have  to  use  a 
large  amount  of  inductance,  and  if  this  inductance  forms 
(as  it  may  easily  do)  the  greater  part  of  the  inductance 
of  the  whole  aerial  circuit, 

we  may  quite  efficiently 
apply  the  detector  across 
the  inductance. 

This  makes  one  of  the 
simplest  and  cheapest 
forms  of  wireless  telegraph 
receivers,  and  is  shown 
diagrammatically  in  Fig. 
72,  where  A  is  the  aerial,  I  FIO.  72. 

the  variable  tuning  induct- 
ance, E  the  earth,  D  the  crystal,  and  T  the  telephones. 

452.  Most  of  the  amateur  stations,  more  especially 
those  in  towns,   have  very  small  aerials  for  obvious 
reasons,  and  as  they  are  chiefly  used  for  "  picking  up  " 
signals  from    stations  using    long  wave-lengths,   this 
form    of    receiver    is    particularly    appropriate.    With 
such  short  aerials  even  the  waves  transmitted  from  ship 
stations  are  sufficiently  long  to  necessitate  the  use  of 


122 


WIRELESS  TELEGRAPHY 


a  comparatively  large  inductance  in  series  with  the 
aerial,  so  that  the  receiver  may  also  be  used  fairly 
efficiently  for  receiving  signals  from  ships. 

THE  POTENTIOMETER 

453.  Some  crystals,  for  example  carborundum, 
become  more  sensitive  to  minute  currents  when  a  slight 
initial  voltage  is  applied  across  them.  This  voltage  must 

Q 

I 


be  regulated  exactly  to  suit  the  particular  crystal  which  is 
being  used,  and  this  regulation  is  accomplished  by  means 
of  a  potentiometer. 

454.  A  potentiometer,  shown  in  Fig.  73,  consists  of 
a  resistance  coil  R,  connected  across  a  battery  Q,  and 
provided  with  a  sliding  contact  S,  by  means  of  which  a 
lead  can  be  connected  to  any  point  along  the  resistance. 

455.  The  resistance  of  the  coil  should  be  kept  suffi- 
ciently high,  so  that  the  current  passing  through  it  from 
the  battery  is  not  sufficient  to  discharge  the  battery 


WIRELESS  TELEGRAPHY  123 

rapidly.  Too  high  a  resistance  becomes  impracticable, 
as  either  the  resistance  wire  with  which  the  coil  is  wound 
would  have  to  be  so  fine  that  it  would  easily  become 
broken  or  cut,  or  the  resistance  coil  would  have  to  be 
of  such  a  length  that  it  would  not  be  convenient  on 
account  of  its  size.  In  practice  suitable  resistance  coils 
can  be  wound  having  a  resistance  of  about  200  ohms, 
and  this  connected  across  a  battery  of  4  volts  will  only 
allow  about  one-fiftieth  of  an  ampere  to  pass  through  it 
(vide  paragraph  78),  so  tliat  a  battery  consisting  of  three 
small  dry  cells  would  be  sufficient  to  maintain  its  voltage 
for  many  weeks  with  continuous  working. 

456.  On  referring  to  diagram  (Fig.  73)  and  assuming 
that  the  voltage  of  the  battery  is  4  volts,  we  have  a  differ- 
ence of  potential  between  the  two  ends  of  the  resistance 
coil,  A  and  B,  of  4  volts  ;  therefore,  if  we  connect  a  wire 
W2  to  the  end  of  the  resistance  coil  A,  and  another  wire 
Wx  to  -the  sliding  contact  S,  and  move  the  latter  to  the 
far  end  of  the  coil  shown  in  dotted  lines  and  marked  S1} 
the  voltage  between  the  two  wires  will  be  4  volts.    If, 
however,  we  slide  the  contact  towards  the  end  of  the  coil 
marked  A,  the  voltage  between  the  two  wires  diminishes 
until  the  voltage  becomes  zero,  when  the  slider  occupies 
the  position  S2.    It  is  obvious  that  the  voltage  across  the 
two  wires  will  be  in  proportion  to  the  distance  the  slid- 
ing contact  is  from  the  point  A,  and  that  by  moving  the 
slider  to  any  point  between  the  two  extreme  ends  of  the 
resistance  we  can  regulate  the  voltage  between  the  two 
wires  to  any  intermediate  value  between  0  and  4  volts. 

457.  With  most  carborundum  crystals,  the  voltage 
which  should  be  applied  across  them  to  bring  them  to 
their  most  sensitive  state  is  somewhere  between  1  and  2 
volts,  so  that  by  applying  this  potentiometer  to  our 


124 


WIRELESS  TELEGRAPHY 


crystal,  we  have  a  simple  means  of  bringing  the  latter  to 
its  most  sensitive  state. 

METHOD  OF  APPLYING  THE  POTENTIOMETER  TO  THE 
CRYSTAL 

458.  The  method  of  applying  the  voltage  obtained 
from  the  potentiometer  to  the  crystal  is  not  as  straight- 
forward as  it  might  at  first  appear  to  be. 

459.  The  most  obvious  way  of  doing  it  is  shown  in 
Fig.  74,  where  the  two  wires  from  the  potentiometer  are 
connected  one  to  either  side  of  the  crystal.     But  it  will 
be  immediately  seen  that  this  entirely  neutralises  the 


I 


FIG.  74. 


FIG.  75. 


value  of  our  crystal  as  a  rectifier,  for  the  oscillating 
currents,  instead  of  trying  to  pass  through  the  crystal 
to  the  telephones,  will  pass  through  the  resistance  of  the 
potentiometer  to  the  telephones. 

460.  We  must  therefore  devise  some  means  of 
applying  the  voltage  to  the  crystal  without  making  a 
bye-pass  for  the  oscillating  currents  induced  in  the 
inductance  coil. 

This  can  be  accomplished  by  connecting  up  the  circuit, 
as  shown  in  Fig.  75,  where  the  junction  of  the  battery 


WIRELESS  TELEGRAPHY  125 

and  the  resistance  coil  is  connected  to  the  earth  side 
of  the  crystal.  One  side  of  the  telephone  is  then 
connected  to  the  earth  terminal,  and  the  other  side  to 
the  sliding  contact  of  the  potentiometer. 

461.  Assuming  that  the   common  junction  of   the 
battery,  resistance  coil,  and  crystal  J  is  the  negative 
side  of  the  battery,  the  sliding  contact  S  is  the  positive, 
and  this  positive  E.M.F.  is  conducted  to  the  other  side 
of  the  crystal  through  the  telephones  and  through  the 
aerial  inductance,  as  indicated  by  the  arrows. 

Thus  it  will  be  seen  that  we  have  accomplished  what 
we  desired,  i.e.  to  apply  an  adjustable  voltage  across  the 
crystal  without  forming  any  short  cut  for  the  oscillating 
currents,  which  must  therefore  pass  through  the  crystal 
and  there  be  rectified  before  they  reach  the  telephones. 

THE  TWO-CIRCUIT  RECEIVER 

462.  As  already  explained,  the  single  circuit  receiver 
just  described  is  quite  efficient  for  stations  that  are 
receiving   comparatively  long   wave  -  lengths  on  short 
aerials,  but  it  would  be  insensitive  for  stations  which 
might  be  required  to  receive  messages  on  wave-lengths 
as  short  as,  or  shorter  than,  the  fundamental  wave-length 
of  the  aerial. 

463.  If  we  can  cause  all  the  energy  in  our  aerial  circuit 
to  be  transferred  to  a  secondary  circuit  and  apply  our 
detector  across  the  whole  of  the  inductance  and  capacity 
of  this  secondary  circuit,  it  is  obvious  that  the  size  of 
the  aerial  will  not  limit  us  as  to  the  value  of  the  wave- 
length for  which  such  a  receiver  can  be  efficiently  used. 

464.  Such   a   receiver   has   two   distinct   oscillatory 
circuits,  both  of  which  must  be  in  tune  with  the  wave- 
length which  it  is  desired  to  receive.     These  two  circuits 


126  WIRELESS  TELEGRAPHY 

are   called  respectively   the   primary   circuit   and   the 
secondary  circuit. 

465.  The  primary  circuit — as  in  the  case  of  the  single 
circuit  receiver — will  consist  of  the  aerial,  an  adjustable 
inductance,  and  an  adjustable  condenser  for  tuning  up 
tjie  circuit,  and  a  primary  coil,  by  means  of  which  the 
oscillations  can  be  induced  into  the  secondary  circuit, 
thus  transferring  the  energy  from  the  primary  to  the 
secondary  circuit  (vide  paragraph  346). 

466.  The  secondary  circuit  will  consist  of  an  induct- 
ance coil  with  a  variable  condenser  connected  across  it, 
by  means  of  which  the  wave-length  of  this  circuit  can  be 
adjusted  so  as  to  be  in  tune  with  the  primary  circuit 
and  at  the  same  time  with^he  wave-length  which  it  is 
desired  to  receive. 

467.  The  inductance  coil  of  this  secondary  circuit 
must  be  so  placed  relatively  to  the  primary  Coil  that  the 
oscillating  currents  occurring  in  the  latter  will  induce 
similar  oscillations  in  the  former ;    that  is  to  say,  the 
axes  of  the  two  coils  must  be  in  line  with  one  another, 
and  the  two  coils  must  be .  sufficiently  close  together 
(vide  paragraph  407). 

468.  These  circuits  are  shown  diagrammatically  in 
Fig.  76,  where  the  primary  oscillatory  circuit  is  formed 
by  A  the  aerial,  I  the  aerial  tuning  inductance,  C  the 
aerial   tuning  condenser,  P   the   primary  coil,  and  E 
the  earth. 

The  secondary  oscillatory  circuit  is  formed  by  S 
the  secondary  coil  and  B  the  secondary  tuning  con- 
denser ;  the  common  axis  of  the  primary  coil  and  the 
secondary  coil  being  denoted  by  the  dotted  line  XY. 

469.  The  method  of  applying  the  potentiometer  to 
the  crystal  in  this  case  is  shown  in  Fig.  77. 


WIRELESS  TELEGRAPHY 


127 


470.  By  applying  our  detector,  as  -shown  in  the 
diagram,  across  the  secondary  inductance  coil,  we  are 
Bpplying  it  in  the  most  efficient  manner  possible, 
since,  no  matter  to  what  wave-length  the  secondary 


FIG.  77. 


circuit  is  adjusted,  the  detector  will  be  applied  to  the 
whole  of  the  inductance  in  that  circuit. 


PROPORTION  OF  INDUCTANCE  AND  CAPACITY  IK 
SECONDARY  OSCILLATORY  CIRCUIT 

471.  Another  point  which  we  have  not  yet  touched 
upon,  affecting  the  efficiency  of  the  crystal  detector,  is 
the  proportion  of  the  inductance  to  the  capacity  of  the 
oscillatory  circuit  to  which  the  detector  is  applied  to 
obtain  maximum  efficiency.  In  addition  to  the  fact  that 
it  is  necessary  that  the  detector  be  connected  across  the 
whole  of  the  inductance,  it  is  found  in  practice  that  the 
greater  the  inductance  of  that  circuit  compared  with  its 
capacity,  the  more  efficient  will  the  crystal,  as  a  detector, 
become. 

The  reason  for  this  is  explained  later  in  paragraph  488, 
but  for  the  time  being  we  must  take  it  as  a  fact  and 
develop  our  receiver  accordingly. 


128  WIRELESS  TELEGRAPHY 

472.  At  first  sight  it  would  appear  that  there  is  no 
limit  to  the  amount  by  which  we  can  increase  the  in- 
ductance of  the  secondary  coil  S,  so  long  as  we  reduce 
the  capacity  of  the  condenser  B  in  proportion.     This, 
howeveij-is  not  the  case,  for  by  increasing  the  number  of 
turns  on  the  inductance  S  we  not  only  increase  the 
inductance  of  the  circuit  but  also  the  capacity. 

473.  Up  to  the  present  we  have  regarded  coils  of  wire 
as  having  only  the  quality  of  inductance.    As  a  matter 
of  fact,  however,  every  coil  of  wire  has  self-capacity,  and 
for  this  reason  it  is  found  that  every  coil  of  wire,  even 
without  a  condenser  connected  across  it,  forms  an  open 
oseillatory   circuit,   and   has   all   the   essentials   of   an 
oscillatory  circuit — that  is  to  say,  the  two  qualities  of 
inductance  and  capacity.     This  self-capacity  then  limits 
the  amount  of  inductance  we  can  use,  for  in  increasing 
the  inductance  we  cannot  avoid  increasing  also  the 
capacity  of  the  circuit. 

474.  The  most  efficient  coil  that  we  can  design  for  the 
secondary  circuit  of  the   crystal  receiver  is  therefore 
one  whose  wave-length  by  itself  will  be  the  required  value 
without    the    addition    of    any    extra    capacity.     Our 
adjustable  condenser,  however,  is  necessary  in  order  to 
enable  us  to  increase  the  wave-length  of  the  secondary 
circuit,  for  a  receiver  only  capable  of  receiving  one  length 
of  wave  would  be  very  inconvenient ;  but  again  we  are 
limited  to  the  extent  to  which  we  can  vary  it  by  the  fact 
that  as  we  increase  the  capacity  across  the  inductance, 
so  do  we  decrease  the  efficiency  of  our  detector  when 
applied  to  that  circuit  (vide  paragraph  471). 

475.  In  practice  it  is  found  that,  without  materially 
affecting  the  efficiency  of  the  detector,  we  can  connect 
a  sufficiently  large  capacity  across  the  inductance  to 


WIRELESS  TELEGRAPHY  129 

increase  its  wave-length  to  about  three  times  its  original 
wave-length.  If  we  go  beyond  this  point  the  reduction 
in  the  efficiency  of  the  detector  becomes  noticeable. 

476.  We  may  say,  then,  that  with  a  two  -  circuit 
receiver  in  which  a  crystal  is  used  as  a  detector,  the 
maximum  efficiency  is  obtained  when  the  capacity  across 
the  secondary  condenser  is  reduced  to  zero.    Further, 
we  may  say  that  the  maximum  wave-length  to  which  it 
can  be  efficiently  tuned  will  be  about  three  times  the 
value  of  its  minimum  wave-length.    Thus,  if  the  shortest 
wave-length  which  a  station  is  required  to  receive  is 
300  metres,  the  receiver  would  be  designed  so  that  the 
minimum  wave-length  to  which  it  can  be  adjusted  will 
be,  300   metres,   and   its   maximum  wave-length  will 
then  be  about  900  metres. 

477.  Where  a  longer  range  of  wave-length  than  this 
is  required,  special  arrangements  have  to  be  made  by 
which  the  secondary  inductance  coil  can  be  changed  ; 
thus  if  a  receiver  is  required  to  receive  wave-lengths  of 
any  value  between  300  and  1500  metres,  it  will  probably 
have  two  secondary  inductance  coils,  one  of  which  will 
allow  the  receiver  to  be  tuned  up  from  300  to  900  metres, 
and  the  other  from,  say,  600  to  1800  metres. 

CHARACTERISTIC  CURVE  OF  CRYSTAL 

478.  Up  to  the  present  we  have  considered  the  action 
of  the  crystal  to  be  purely  one  of  rectifying  the  oscil- 
latory currents  induced  across  it  into  uni-directional 
currents. 

479.  The  crystal  can  be  better  considered  as  a  con^ 
ductor  offering  a  certain  resistance  to  current  passing 
through  it  in  one  direction,  and  offering  a  very  much 

K 


130 


WIRELESS  TELEGRAPHY 


larger  resistance  to  current  trying  to  pass  through  it  in 
the  other  direction. 

480.  Its  value  as  a  sensitive  detector,  however,  de- 
pends upon  another  property.     Even  in  the  direction  of 
conductivity,  the  crystal  does  not  act  in  the  same  way  as 
an  ordinary  conductor. 

481.  With  an  ordinary  conductor  the  current  passing 
through  it  increases   directly   as   the   voltage   applied 


across  it  increases  (vide  paragraph  78).  Thus,  if  we  draw 
a  "  curve "  illustrating  the  increase  in  the  current 
which  would  flow  through  an  ordinary  conductor  as  the 
voltage  across  it  is  increased,  this  curve  would  take  the 
form  of  a  straight  line,  as  shown  in  Fig.  78. 

482.  If  a  curve  be  drawn  illustrating  the  increase  in 
the  current  passing  through  a  crystal  as  the  voltage  across 
it  is  increased,  it  will  take  the  form  shown  in  Fig.  79. 

483.  In  this  case  it  will  be  noticed  that  when  the 
voltage  is  increased  beyond  the  point  A,  the  current 


WIRELESS  TELEGRAPHY 


131 


passing  through  it  rises  very  much  more  rapidly  than 
before  in  proportion  to  the  increase  in  voltage  across  it. 

484.  This  is  due  to  the  fact  that  the  effective  resistance 
of  the  crystal  does  not  remain  constant,  but  starts  to 
decrease  when  the  voltage  across  it  is  increased  above  a 
certain  value. 

485.  By  referring  to  this  curve  it  will  be  seen  that 


between  the  points  0  and  A  a  certain  increase 
in  the  voltage  across  the  crystal  produces  a  very  small 
increase  in  the  current  passing  through  .it,  and  thus 
through  the  telephones,  whereas  beyond  the  point  A 
the  same  increase  in  the  voltage  across  the  crystal  pro- 
duces a  larger  increase  in  the  current  passing  through  it. 
486.  To  produce  a  sound  in  the  telephone  it  is  neces- 
sary that  the  current  passing  through  it  be  increased, 
and  the  strength  or  loudness  of  that  sound  will  depend 
upon  the  amount  by  which  the  current  is  increased. 


132  WIRELESS  TELEGRAPHY 

487.  It  is  obvious,  therefore,  that  the  voltage  pro- 
duced across  the  secondary  coil  of  the  jigger  by  the 
oscillating  currents  will  cause  a  greater  increase  in  the 
current  passing  through  the  crystal  and  telephones  if  it 
be  applied  after  the  point  A  is  reached.    It  is  for  this 
reason  that  a  potentiometer  is  necessary  in  order  to 
bring  the  initial  voltage  across  the  crystal  up  to  the 
point  A. 

488.  In  paragraph  471  we  mentioned  the  fact  that 
the  efficiency  of  the  receiver  depended  on  keeping  the 
capacity  of  the  secondary  circuit  as  small  as  possible. 
The  reason  for  this  is  that  a  given  amount  of  energy  will 
produce  a  greater  increase  in  voltage  across  a  small 
condenser  than  across  a  large  condenser  (vide  paragraph 
272). 

Now  in  our  receiver  the  energy  in  the  circuit  is 
a  fixed  quantity,  depending  upon  the  strength  of  the 
oscillations  produced  in  the  aerial ;  this  energy  is  in 
turn  transferred  to  the  secondary  circuit. 

It  is  therefore  obvious  that  the  only  way  to  increase 
the  voltage  across  the  condenser  is  to  reduce  the  value 
of  that  condenser. 

489.  In  so  reducing  it  we  reduce  also  the  wave-length 
of  that  circuit,  and  as  it  is  necessary  to  keep  this  in  tune 
with  the  wave-length  which  is  being  received,  we  must 
counterbalance  the  effect  of  reducing  the  capacity  by 
increasing  the  inductance  of  the  circuit.     The  extent  to 
which  we  can  do  this,  as  already  explained,  is  limited  by 
the  fact  that  every  coil  of  wire  has  self-capacity,  and  as 
we  increase  the  coil  to  get  a  greater  inductance,  so,  at 
the  same  time,  we  increase  its  capacity. 

490.  The  rate  at  which  we  increase  this  self-capacity, 
however,  can  be  controlled,  to  a  large  extent,  by  the 


WIRELESS  TELEGRAPHY  133 

design  of  the  coil — that  is  to  say,  by  its  diameter,  its 
length,  and  the  size  of  the  wire  with  which  it  is  wound. 

491.  A  question  which  will  probably  arise  in  the 
minds  of  those  studying  this  explanation  will  be  that, 
in  describing  the  receiver,  we  said  that  the  crystal  was 
placed  across  the  inductance,  whereas  in  explaining  the 
reason  for  keeping  this  inductance  high,  in  proportion 
to  the  capacity,  we  take  the  point  of  view  that  we  wished 
to  increase  the  voltage  across  the  condenser.     This  is 
only  because  it  is  easier  to  understand  how  the  voltage 
must  necessarily  increase  across  the  condenser  if  the 
value  of  that  condenser  is  decreased,  and,  since  in  an 
oscillatory  circuit  the  condenser  is  connected  across  the 
inductance,  it  follows  that  the  voltage  across  the  in- 
ductance is  likewise  increased. 

THE  TELEPHONE  RECEIVER 

492.  So  far  we  have  not  touched  upon  the  construction 
of  the  telephone  receivers. 

The  function  of  the  telephone  receivers  (usually 
called  "  telephones  "  for  short)  is  to  convert  electric 
currents  into  an  audible  sound. 

It  is  of  course  of  as  much  importance  for  this  part 
of  the  apparatus  to  be  efficient  as  any  other,  and  in  order 
to  be  efficient  it  must  be  made  suitable  for  the  circuit 
to  which  it  is  applied. 

493.  A  telephone  receiver  consists  essentially  of  an 
electro-magnet  and  a  diaphragm. 

The  diaphragm  is  a  circular  piece  of  very  thin  sheet 
iron,  supported  all  round  the  edge  by  the  outer  case,  or 
shell,  of  the  "  ear-piece,"  as  close  to  the  face  of  the 
magnets  as  possible  without  actually  touching. 


134 


WIRELESS  TELEGRAPHY 


494.  Fig.  80  shows  diagrammatically  a  section  of  a 
telephone  ear-piece  where  A  is  the  iron  core  of   the 
electro-magnet,  B  the  coils  of  the  electro-magnet,  C  the 
case,  or  shell,  and  D  the  diaphragm. 

495.  Unlike  an   ordinary  electro -magnet,  the   iron 
core  of  the  telephone   receiver  is  to  a  certain   extent 
permanently  magnetised. 

496.  It  is  evident,  then,  that   the  diaphragm   will 


normally  be  strained  slightly  towards  the  magnet,  as 
shown  by  the  full  line  D,  in  Fig.  80. 

As  already  mentioned,  it  is  supported  by  the  shell  of 
the  ear-piece  all  round  its  edge,  but,  being  thin  and 
springy,  it  will  bulge  in  the  middle  towards  the  magnet. 

497.  The  action  of  the  telephone  receiver  is  as  follows  : 
If  a  current  is  sent  through  the  coils  in  such  a  direction 
that  the  lines  of  force  set  up  by  it  assist  those  of  the 
permanent  magnet,  the  strength  of  the  magnet  will  be 
increased,   and   the  diaphragm   will'  be  attracted  still 
closer  to  the  magnet,  thus  taking  the  position  shown 
by  the  dotted  line  Dx. 

498.  If,  on  the  other  hand,  a  current  is  sent  through 


WIRELESS  TELEGRAPHY  135 

the  coil  in  the  opposite  direction,  thus  setting  up  lines 
of  force  opposing  those  of  the  permanent  magnet,  the 
strength  of  the  magnet  will  be  decreased  and  the  dia- 
phragm will  be  allowed  to  spring  farther  away  from  the 
pole  and  take  up  the  position  shown  by  the  dotted  line 
D2,  owing  to  the  fact  that  it  has  already  been  displaced 
out  of  its  normal  position  due  to  the  normal  pull  of  the 
permanent  magnet. 

499.  Owing  to  the  form  of  the  diaphragm,  it  acts  in 
just  the  same  way  as  the  head  of  a  drum,  and  will 
produce  a  big  sound  with  a  comparatively  small  dis- 
placement of  its  centre. 

Just  as  the  noise  produced  by  a  drum  will  depend 
upon  how  hard  it  is  hit  by  the  drumstick,  so  will  the 
noise  produced  by  the  diaphragm  depend  upon  the 
amount  of  increase  or  decrease  in  the  magnetisation  of 
the  magnet. 

HIGH  RESISTANCE  TELEPHONES 

500.  It  is  obvious  that  the  increase  or  decrease  in 
the  magnetism  of  the  magnet  will  depend  upon  the 
magnetisation  force  or  "  magneto-motive  force  "  which 
is  applied  to  it. 

The  magneto-motive  force  depends  upon  two  things  : 
(1)  the  number  of  turns  of  wire  which  are  encircling  the 
magnet,  and  (2)  the  amount  of  current  passing  through 
them  (vide  paragraph  96). 

501.  For  a  given  size  of  magnet  we  have  only  a 
definite  space  into  which  to  get  our  turns  of  wire,  so  that 
the  only  way  of  increasing  the  number  of  turns  we  can 
wind  on  the  magnet  is  to  decrease  the  size  of  the  wire. 
The  thinner  the  wire  the  greater  the  number  of  turns 
which  we  can  get  into  the  space  at  our  disposal. 


136  WIRELESS  TELEGRAPHY 

502.  Unfortunately,  however,  as  we  reduce  the  size 
of  the  wire,  so  do  we  increase  the  resistance  per  turn  of 
that  wire,  and  therefore  decrease  the  amount  of  current 
which  would  pass  through  it  for  a  given  voltage.    There- 
fore, unless  the  current  at  our  disposal  is  already  limited 
by  some  external  resistances,  we  shall  not  gain  anything 
by  increasing  the  number  of  turns  if  at  the  same  time  we 
increase  the  resistance  of  the  coil  in  proportion. 

503.  If,  however,  the  telephone  is  in  a  circuit  in  which 
there  is  already  a  high  resistance,  then  the  increase  in 
the  resistance  of  the  coil  will  not  have  so  great  an  effect 
on  the  total  resistance  of  the  circuit,  and  therefore  on 
the  current  which  is  passing  through  that  circuit. 

504.  For  an  example,  let  us  suppose  that  a  coil  wound 
with  10  turns  of  a  certain  size  wire  will  have  a  resistance 
of  1  ohm,  and  let  us  suppose  that  the  external  resistance 
of  the  circuit  in  which  the  coil  is  connected  is  99  ohms. 
The  total  resistance  of  the  circuit  is  then  100  ohms.    If 
our  voltage  across  this  circuit  is  1  volt,  then  it  follows 
that  our  current  through  this  resistance  will  be  a  one- 
hundredth  part  of  an  ampere,  and  consequently — 

Magneto-motive  force  =  y^'x  10  turns  =  ^. 

505.  Now  let  us  wind  the  same  coil  with  wire  y^th 
the  former  cross-sectional  area.    It  follows  that  we  shall 
get  10  times  the  number  of  turns — that  is  to  say,  wetahall 
get  100  turns  of  wire  on  to  the  coil,  but  our  resistance 
per  turn  will  be  increased  ten  times.    The  resistance  per 
turn  in  the  first  coil  was  y\jth  of  an  ohm,  so  that  our 
resistance  per  turn  will  now  be  1  ohm  ;    therefore  the 
resistance  of  the  coil  will  be  100  ohms. 

506.  Adding  this  to  our  external  resistance  we  get 
a  total  resistance  in  the  circuit  of  199  ohms.    Now 


WIRELESS  TELEGRAPHY  137 

for  the  same  voltage,  i.e.  1  volt  across  this  circuit, 
we  shall  get  xd9tn  of  an  ampere,  and  therefore  in 
this  case — 

Magneto-motive  force  =  T-^  x  100  =  approx.  J. 

507.  It  is  obvious,  therefore,  that  in  this  case  we 
have  increased  our  magneto-motive  force  nearly  five 
times  by  winding  the  coils  with  a  finer-sized  wire. 

508.  On  examining  the  diagrams  of  connections  of 
our  wireless  telegraph  receiver,  it  will  be  seen  that  any 
current  passing  through  the  telephones  will  have  to  pass 
through  the  crystal. 

509.  The  resistance  of  our  crystal  at  its  most  sensitive 
point  is  of  the  order  of  10,000  ohms.     It  will  there- 
fore  be   obviously  inefficient  to  wind    the  telephone 
receiver  with  such  a  sized  wire  that  its  resistance  is 
only,  say,  200  ohms,  if  a  finer  wire  is  available. 

510.  In  practice  special  telephones  are  made  suitable 
for    circuits    with    such    external    resistances.    These 
telephones  are  wound  with  the  very  finest  wire  which  it 
is  possible  to  manufacture,  in  order  to  get  the  greatest 
possible  number  oi  turns  on  to  the  limited  space  of  the 
bobbins. 

511.  Such  telephones    are  called   High   Resistance 
Telephones,   and  have  a  resistance  of  approximately 
3500  ohms  per  ear-piece,  and  two  ear-pieces  can  be  used, 
connected  in  series,  thus  making  a  total  resistance  of  a 
pair  of  telephones  about  7000  ohms. 

512.  The  point  which  must  be  clearly  understood  is 
that  the  object  of  using  high  resistance  telephones  is  not 
because  they  have  a  high  resistance,  but  because  they 
are  wound  with  a  very  much  larger  number  of  turns  than 
the  low  icsistance  telephones,  and  therefore,  owing  to 


138  WIRELESS  TELEGRAPHY 

the  high  external  resistance  of  the  circuit,  the  magneto 
motive  force  is  increased  to  a  greater  extent  than  it  is 
decreased  by  the  increase  of  resistance  of'  that  circuit. 

RECTIFYING  PROPERTIES  OF  CARBORUNDUM 

513.  In  paragraph  485  we  explained  why  it  is  necessary 
to  adjust  the  initial  voltage  across  the  carborundum 
crystal  to  a  certain  value  in  order  that  a  given  increase 
in  voltage  will  cause  the  greatest  possible  increase  in 
the  current  passing  through  it.    We  have  not,  however, 
explained  why  it  is  necessary  to  adjust  the  initial  volt- 
age across  the  crystal  to  the  exact  point  where  the 
effective  resistance  of  the  crystal  commences  to  decrease 
rapidly. 

514.  Referring  again  to  the  characteristic  curve  of  a 
carborundum  crystal  shown  in  Fig.  79,  although  it  is 
obvious  that  the  crystal  will  be  more  sensitive  when  the 
point  A  is  reached,  it  is  not  quite  so  obvious  why  it  is 
necessary  to  adjust  the  initial  voltage  across  the  crystal 
exactly  to  the  point  A,  and  not  to  any  point  beyond  it, 
such  as  the  point  B  shown  in  Fig.  81. 

515.  As  can  be  seen  from  this  curve,  a  given  increase 
in  voltage  will  cause  practically  the  same  increase  in 
current  passing  through  the  crystal  whether  this  increase 
be  applied  at  the  point  B  or  the  point  A,  but  we  must 
remember   that   the   extra   voltage   provided   by    the 
oscillatory  current  in  the  secondary  of  the  jigger  is  an 
alternating  current  voltage,  that  is  to  say,  a  voltage 
varying  from  a  positive  value  at  one  instant  to  a  negative 
value  at  the  next  instant. 

516.  Since   the   initial   voltage   applied   across    the 
crystal  is  a  direct  current  voltage  obtained  from  the 


WIRELESS  TELEGRAPHY  139 

potentiometer,  it  follows  that  the  alternating  current 
voltage  will  at  one  instant  be  assisting  the  direct  current 
voltage,  and  at  the  next  instant  opposing  it. 

To  facilitate  explanation,  let  us  put  these  voltages 
into-  figures. 

517.  Let  us  suppose  that  the  initial  voltage  across 
the  crystal  to  bring  it  up  to  the  ppint  A  is  2  volts,  and 
the  voltage  required  to  bring  it  up  to  the  point  B  is 
2J  volts,  these  voltages  being  positive  volts. 

518.  Let  us  also  suppose  that  the  value  of  the  alter- 
nating   voltage    provided    by    the    oscillating    current 
varies  from  minus  ^  a  volt  to  plus  \  a  volt ;  it  is  obvious, 
then,   that  during  the  time  that  the  oscillations  are 
being  received  the  resulting  voltage  across  the  crystal, 
if  the  initial  voltage  be  adjusted  to  the  point  A,  will  vary\ 
from  1|  volts  to  2|  volts.     Similarly,  if  the  initial  voltage 
across  the  crystal  be  adjusted  to  the  point  B,  the  resulting 
voltage  will  vary  from  If  volts  to  2f  volts. 

519.  Now  let  us  draw  two  separate  curves,  shown  in 
Figs.  82  and  83.  showing  the  result  of  this  variation  in 
voltage  on  the  current  passing  through  our  telephones, 
taking  our  figures  from  the  curve  shown  in  Fig.  81. 

The  curve  in  Fig.  82  shows  the  resulting  current  in 
the  telephones  when  the  initial  voltage  of  the  crystal  is 
adjusted  to  point  A. 

520.  At  this  point  the  value  of  the  current  passing 
through  the  crystal  and  telephones  before  any  oscillations 
are  produced  in  the  secondary  circuit  is  1,  therefore  we 
may  draw  a  heavy  line  DD,  representing  the  normal 
value  of  the  current. 

521.  When  the  negative  part  of  the  first  oscillation  is 
applied  across  the  crystal,  the  result,  as  already  explained, 
is  to  reduce  the  voltage  to  1^  volts,  thus  the  current 


140 


WIRELESS  TELEGRAPHY 


passing  through  the  telephones  will  be  reduced,  but,  as 
will  be  seen  by  referring  to  Fig.  81,  owing  to  its  being  on 


the  flat  part  of  the  curve,  the  reduction  in  the  amount 


of  current  passing  through  the  telephones  is  extremely 


Via.  82. 


small ;   by  reading  from  the  curve  we  find  it  is  reduced 
to  the  value  of  f . 

Therefore  the  curve  representing  the  actual  current 


WIRELESS  TELEGRAPHY 


ui 


passing  through  the  crystal  and  telephones  when  the 
negative  part  of  the  first  oscillation  is  applied,  will  dip 
just  below  the  line  DD. 

522.  The  next  half  of  the  oscillation  is  positive,  and 
therefore  has  the  result  of  increasing  the  voltage  across 
the  crystal  to  2|  volts. 

By  referring  again  to  Fig.  81,  it  will  be  seen  that  the 
effect  on  the  value  of  the  current  is  to  increase  it  to  4. 

We  may  therefore   continue   our  current  curve  in 


.  83. 


Fig.  82,  which  will  now  show  the  current  rising  to  the 
value  4  above  the  normal  line  DD. 

523.  A  similar  cycle  will  take  place  for  each  oscillation, 
with  the  result  that  we  get  a  series  of  high  peaks  above 
the  normal  current  line,  and  a  series  of  very  shallow 
dips  below  this  line. 

524.  These  oscillations  are  taking  place  at  the  rate 
of  perhaps  millions  per  second,  according  to  the  length 
of  wave  which  is  being  received. 

If  the  wave-length  received  is  100  feet,  the  number  of 
oscillations  per  second  will  be  10  millions  per  second 
(vide  paragraphs  167  and  174). 


142  WIRELESS  TELEGRAPHY 

525.  These  variations  are  infinitely  too  rapid  for  the 
diaphragm  of  the  telephone  to  follow,  and  it  will  therefore 
be  deflected  to  an  extent  corresponding  to  the  average 
value  of  the  current  passing  through  its  coils. 

526.  Referring  to  Fig.  82  the  average  current  passing 
through   the  telephones  when  the    oscillating  voltage 
is  applied  across  the  crystal  is   shown  by  the  dotted 
line  AA,  drawn  approximately  half-way  between  the 
highest  and  lowest  point  on  the  curve.     This  value  is. 
somewhere  about  2J,  and  the  sound  produced  in  the 
telephones  will  be  proportional  to  the  difference  between 
the  normal  current  passing  through  the  telephones  and 
the  increased  current  due  to   the  oscillating  voltage 
applied,  i.e.  the  difference  between  1  and  2J. 

527.  Now  let  us  see  what  takes  place  if  we  adjust  the 
crystal  to  the  point  B  ;  obtaining  our  values  of  current 
as  before  from  the  curve  in  Fig.  ,81,  we  find  that  the 
result  of  the  oscillating  voltage  is  to  vary  the  value  of 
the  current  from  ^  to  5|,  and  we  may  therefore  draw  a 
new  curve  as  shown  in  Fig.  83,  representing  this  variation 
in  the  value  of  the  current. 

528.  Again,  these  variations  in  current  are  too  rapid 
for  the  diaphragm  of  the  telephone  to  follow,  so  that  it 
will  again  be  deflected  to  an  extent  corresponding  to 
the  average  value  of  this  current. 

529.  The  average  value  of  this  current  will  be  about 
3,  so  that  we  may  draw  a  dotted  line  BB,  representing 
the  average  value  of  the  current  passing  through  the 
telephones  when  the  oscillating  voltage  is  applied  across 
the  crystal. 

530.  But  we  have  already  increased  the  nonnaj  Value 
of  the  current  passing  through  the  telephones  to  the 
value  of  2$,  as  shown  by  the  line  DD,  Fig.  83,  this  being 


WIRELESS  TELEGRAPHY  143 

the  current  which  will  pass  through  the  crystal  and 
telephones  when  the  initial  voltage  of  2J  volts  is  applied 
to  bring  it  up  to  the  point  B,  Fig.  81. 

531.  The    strength    of   the    sound    produced   in   the 
telephones  will  be  proportional,  not  to  the  total  current 
passing  through  the  telephones,  but  to  the  difference  be- 
tween the  current  passing  through  them  when  no  oscilla- 
tions are  being  received  and  the  average  current  passing 
through  them  when  the  oscillations  are  being  received. 

532.  When  the  crystal  was  adjusted  to  the  point  A, 
this  difference  in  current  was  the  difference  between 
1  and  2f ,  so  that  the  strength  of  the  sound  was  propor- 
tional to  1| ;  but  when  the  crystal  was  adjusted  to  the 
point  B,  the  strength  of  the  sound  was  proportional  tc 
the  difference  between  2|  and  3,  which  is  only  £. 

RELATION  BETWEEN  THE  SPARK  FREQUENCY  OF  THE- 
TRANSMITTER  AND  SOUND  PRODUCED  IN  THE 
TELEPHONES  OF  RECEIVER 

533.  In   paragraph  433,   describing  the  wavemeter 
(which  is  in  reality  a  simple  form  of  tuned  receiver), 
we  said  that  the  current  produced  in  the  telephone 
receiver  would   be   an   intermittent   current,  and  the 
number  of  interruptions  per  second  would  be  the  same 
as  the  number  of  sparks  per  second  in  the  oscillatory 
circuit  which  is  being  measured. 

The  explanation  of  this  is  easy  to  follow  if  the  fore- 
going paragraphs  are  thoroughly  understood. 

It  is  obvious  that  the  average  current  passing  through 
the  telephone  from  any  group  of  oscillations  may  be 
regarded  as  a  direct  current  flowing  so  long  as  the 
oscillations  arc  maintained. 


144  WIRELESS  TELEGRAPHY 

534.  If,  then,  the  transmitting  station  were  sending 
out  a  stream  of  continuous  waves  (vide  paragraph  213), 
so  long  as  the  manipulating  key  were  kept  depressed 
we  should  get  a  continuous  current  flowing  through  the 
receiver,  without  interruption. 

As  a  matter  of  fact,  however,  when  we  depress  the 
manipulating  key  we  get  a  succession  of  short  groups  of 
damped  waves,  one  group  each  time  the  condenser  is 
charged  by  the  induction  coil  and  discharged  through 
the  spark  gap. 

535.  The  uni-directional  current,  therefore,  produced 
in  the  telephone  of  the  receiver  will  only  be  maintained 
for  the  time  that  the  group  of  waves  lasts,  with  the  result 
that  the  diaphragm  of  the  telephone  is  deflected  for  an 
instant  only,  and  returns  to  its  normal  position  until 
another  group  of  waves  is  received ;  thus  a  single  click 
will  be  produced  by  each  group  of  oscillations. 

536.  As  each  spark  in  the  transmitter  produces  a 
group  of  waves,  so  does  each  group  of  waves  in  the 
.-eceiver  produce  a  click  in  the  telephones.     Thus  the 
sound  produced  in  the  telephones,  or,  in  other  words,  the 
frequency  of  the  clicks  in  the  telephones,  will  correspond 
with  the  spark  frequency  of  the  transmitter. 

To  TUNE  A  RECEIVER 

537.  We  will  suppose  that  our  receiver  is  of   the 
two-circuit  type — that  is  to  say,  that  it  has  a  primary 
circuit    and    a    secondary     circuit,     both    of     which 
must  be  in  tune  with  the  wave-length  it  is  desired  to 
receive.     The  only  means  we  have  of  telling  whether  the 
receiver  is  in  tune  is  by  the  strength  of  the  signals  in  the 
telephones.     If  either  circuit  of  the  receiver  is  out  of  tune, 
the  signals  are  weakened,  so  that  provided  we  have  a 


WIRELESS  TELEGRAPHY  145 

variable  inductance  or  condenser  in  each  circuit,  and 
provided  we  can  hear  at  least  weak  signals  in  the  tele- 
phones, it  is  a  simple  matter  to  tune  up  the  receiver 
by  listening  to  the  strength  of  the  signals  and  adjusting 
first  one  circuit  and  then  the  other  circuit,  until  the 
sound  is  at  its  loudest. 

538.  If,  however,  we  are  so  much  out  of  tune  to  begin 
with  that  the  signals  are  inaudible,  the  difficulty  of 
tuning  up  is  increased  enormously. 

539.  In  the  case  of   a  single  -  circuit    receiver  the 
difficulty  is  not  so  great,  for  we  have  only  one  circuit  to 
adjust,  and  therefore  we  can  vary  it  slowly  from  its 
maximum   wave-length  to   its   minimum   wave-length, 
and  consequently  we  are  bound  to  pass  the  point  where 
the  receiver  is  in  tune  with,  and  therefore  will  respond 
to,  the  signals. 

540.  In  the  case  of  a  two-circuit  receiver,  however, 
if  signals  are  inaudible  to  begin  with,  we  have  no  means 
of  telling  which  circuit  is  out  of  tune  or  when  the  two 
circuits  are  in  tune  with  each  other. 

541.  If  we  know  the  wave-length  of  the  signals  we 
wish  to  receive,  and  we  have  an  instrument  close  to  our 
receiver  which  can  be  made  to  emit  a  similar  wave-length, 
the  process  of  tuning  up  becomes  quite  simple. 

Such  an  instrument  is  called  a  tuning  buzzer. 

542.  Since  our  detector  and  telephones  are  actuated 
by  the  secondary  circuit  of  the  receiver,  we  should  first 
cause  the  tuning  buzzer  to  induce  waves  into  the  second- 
ary circuit  only,  and  we  should  then  adjust  this  circuit 
until  the  buzzer  signals  in  the  telephones  are  at  their 
loudest. 

Having  accomplished  this,  we  should  next  move  the 
tuning  buzzer  to  a  point  remote  from  the  secondary 

L 


146  WIRELESS  TELEGRAPHY 

circuit,  but  close  to  some  part  of  the  primary  or  aerial 
circuit,  so  that  no  oscillations  can  be  induced  from  it 
directly  into  the  secondary  circuit,  but  only  through 
the  primary  circuit. 

543.  Now  if  the  primary  circuit  is  very  much  out  of 
tune  with  the  wave-length  emitted  by  the  tuning  buzaer, 
it  will  not  respond  to  the  oscillations,  and  therefore  no 
oscillations  will  be  induced  in  the  secondary  circuit, 
but  if  we  vary  the  wave-length  of  the  primary  circuit, 
we  shall  reach  a  point  when  it  is  in  tune  with  the  wave 
emitted  by  the  tuning  buzzer.     Oscillations  will  then 
be  induced  in  the  primary  circuit,  which  will  in  turn 
induce    oscillations    in    the   secondary  circuit,   as   the 
secondary  circuit  has  already  been  tuned  to  the  same 
wave-length.     Thus  when  by  varying  the  adjustment 
of  the  primary  circuit  we  reach  a  point  when  the  signals 
in  the  telephones  are  again  at  their  loudest,  we  know  that 
we  have  reached  the  point  when  the  primary  circuit  is 
in  tune  with  the  tuning  buzzer,   and  therefore  both 
circuits  are  in  tune  with  the  wave-length  emitted  by 
the  tuning  buzzer^ 

THE   TUNING  BUZZER 

544.  The  essentials  of  a  tuning  buzzer  are,  therefore, 

(1)  that  it  can  be  caused  to  emit  feeble  oscillations,  and 

(2)  that  the  frequency  of  these  oscillations  can  be  adjusted 
to  any  predetermined  value. 

545.  To    accomplish    these    desiderata,    the    tuning 
buzzer  has  two  circuits  :    firstly,  an  oscillatory  circuit, 
consisting  of   an   inductance   coil   with  an   adjustable 
condenser,  and,  secondly,  a  generating  circuit,  by  which 
the  oscillatory  circuit  is  excited. 


WIRELESS  TELEGRAPHY 


147 


546.  The  construction  of  the  oscillatory  'circuit  of  a 
tuning  buzzer  is  identical  with  that  of  the  wavemeter, 
which  was  described  in  paragraph  417.    It  consists  of 
a    fixed  inductance  coil  connected  in  series  with  an 
adjustable  condenser,  the  latter  being  provided  with 
a   scale  and   pointer   by   means   of  which   the  value 
of  the  wave-length  to  which  that  circuit  is  adjusted  is 
indicated. 

547.  There  are  several  ways  in  which  this  circuit  can 
be  excited.      We  can,  of   course,  charge  up  the  con- 
denser by  means  of  an  induction  c,oil  and  discharge 
it   through  a  spark  gap  in  the  oscillatory  circuit,  as 


FIG.  84. 

described  in  paragraph  319;  but  this  would  be  an 
expensive  method  and,  moreover,  would  produce  very 
much  stronger  signals  than  are  necessary. 

548.  The  method  most  commonly  used  is  shown 
diagrarnmatically  in  its  simplest  form  in  Fig.  84,  where 
L  is  the  inductance  coil,  C  the  condenser  forming  the 
oscillatory  circuit,  and  where  B  is  a  battery  connected 
across  the  inductance  coil  through  the  contact  S.  If  the 
contact  S  is  depressed,  thus  completing  the  circuit  from 
the  battery  through  the  inductance  coil,  a  continuous 
current  will  flow  through  this  coil.  If  this  circuit  is 
broken  by  releasing  the  contact  the  current  will  be 
instantaneously  interrupted. 


148  WIRELESS  TELEGRAPHY 

549.  As    already   described    in    paragraph    62,    the 
property   of  inductance  is  similar  to  the  mechanical 
.property  of  momentum,  and  therefore,  when  the  current 
is  suddenly  interrupted,  the  energy  due  to  its  momentum 
is  liberated,  and  is  expended  in  the  oscillatory  circuit 
of  which  this  inductance  forms  a  part.     The  result  is 
practically  to  give  this  circuit  a  kick,  causing  it  to 
oscillate  to  its  own  natural  frequency  ;  thus  every  time 
the  battery  circuit  is  broken  we  produce  a  group  of 
oscillations  in  the  oscillatory  circuit  corresponding  to  the 
wave-length  to  which  that  circuit  is  adjusted. 

550.  If  a  battery  of  only  two  or  three  volts  be  used 
and  the  inductance  included  in  the  battery  circuit  be 
a  reasonable  amount,  the  oscillations  set  up  will  be 
sufficiently  strong  to  affect  our  receiver  circuit. 

551.  This,  however,  is  not  quite  sufficient  to  enable 
us  conveniently  to  tune  up  the  receiver,  for  each  group 
of  waves  will  only  give  a  single  click  in  the  telephones. 
Some   automatic   arrangement   must   be  used  to  make 
and  break  the  circuit  rapidly  in  order  to  produce  a  con- 
tinuous buzz  or  note  in  the  telephones,  it  being  very 
much  easier  to  distinguish  when  a  buzz  or  note  reaches 
its  maximum  strength  than  if  only  a  number  of  single 
clicks  were  audible. 

552.  One  method  by  which  this  rapid  making  and 
breaking  of  the  circuit  can  be  accomplished  is  shown 
diagrammatically  in  Fig.  85,  where  the  battery  circuit  B, 
through  the  inductance  L,  is  made  through  a  pair  of 
contacts  S,  S,  one  of  which  is  mechanically  connected  to 
the  armature  A  of  an  ordinary  electric  buzzer  D,  so  that 
when  this  armature  vibrates  it  causes  the  extra  pair  of 
contacts  S,  S  alternately  to  make  and  break  the  battery 
Circuit  through  the  inductance  L. 


WIRELESS  TELEGRAPHY 


149 


553.  In  this  case  two  batteries  are  required,  one  for 
working  the  buzzer  and  the  other  for  the  oscillatory 
circuit. 

554.  There  is  no  reason,  however,  why  the  ordinary 
single  contact  buzzer  cannot  be  used  for  exciting  an 
oscillatory  circuit,  for  by  connecting  it  in  such  a  way 
that  the  current  passing  through  the  coils  of  the  buzzer 
is  made  to  pass  also  through  the  inductance  of  the 
oscillatory  circuit,  as  shown  in  Fig.  86,  we  have  practi- 
cally the  same  conditions  as  before.     Energy  will  be 
stored  up  in  the  inductance  L,  while  the  current  is  passing 


i 


FIG.  85. 


through  it,  and  will  be  liberated  as  soon  as  it  is  inter- 
rupted at  the  contacts  S,  the  energy  thus  liberated 
giving  the  oscillatory  circuit  a  kick,  and  thus  causing  it 
to  oscillate  to  its  own  natural  frequency. 

555.  In  this  case,  however,  when  the  contacts  S  are 
broken,  we  not  only  liberate  the  energy  stored  up  in  the 
inductance  L,  but  we  also  liberate  the  energy  which  is 
stored  up  in  the  inductive  coils  of  the  buzzer  itself. 

The  inductance  of  these  coils  is  m#ny  times  greater 
than  the  inductance  in  the  oscillatory  circuit,  and 
therefore  a  very  much  larger  amount  of  energy  will  bo 
liberated  at  this  point  when  the  circuit  is  interrupted. 

556.  If  no  path  is  provided  in  which  this  energy  can 


150 


WIRELESS  TELEGRAPHY 


dissipate  itself,  it  will  form  a  small  arc  at  the  contacts  S, 
and  dissipate  itself  gradually  in  this  manner. 

557.  Unfortunately,,  this  arc  will  also  form  a  path 
for  the  energy  stored  up  in  the  inductance  L,  with  the 
result  that  the  energy  will  be  dissipated  in  the  same 
way  without  charging  up  the  condenser ;  so  that  under 
these  conditions  the  oscillatory  circuit  would  not  be 
excited. 

558.  If,  however,  we  connect  a  non-inductive  resist- 
ance, as  shown  by  R,  Fig.  86,  across  the  coils,  of  the 


buzzer,  the  energy  liberated  from  the  buzzer  coils  will 
expend  itself  in  the  circuit  formed  by  the  coils  of  the 
buzzer  and  the  resistance  coil  R,  instead  of  forming  an 
arc  at  the  contact  S.  But  the  energy  liberated  from 
the  inductance  L  will  not  have  this  circuit  in  which  to 
expend  itself,  for  the  circuit  is  interrupted  at  the  contact 
S,  and  will  therefore  have  to  expend  itself  in  charging 
up  the  condenser  C. 

559.  The  connections  shown  in  the  figure  are  those" 
usually  adopted  in  an  ordinary  tuning  buzzer,  and  it  is 
obvious  that  the  instrument  can  be  used  either  to  buzz 


WIRELESS  TELEGRAPHY  151 

a  calibrated  closed  oscillatory  circuit,  and  so  make  it 
emit  waves  of  any  desired  length  for  the  purpose  of 
testing  receivers,  etc.,  or  it  can  be  used  to  buzz  any  other 
oscillatory  circuit  in  order  that  the  wave-length  of  that 
circuit  may  be  measured  by  means  of  a  wavemeter. 

560.  We  have  now  explained  briefly  the  principles 
of  the  design  and  application  of  crystal  receivers.  There 
are,  however,  many  other  forms  of  receiver,  some  of  which 
make  use  of  phenomena  entirely  different  from  those 
already  explained. 

The  purpose  of  this  book  will  be  served  by  describing 
only  two  other  forms  of  receiver,  namely,  the  Electrolytic 
Detector  and  the  Magnetic  Detector. 


THE  ELECTROLYTIC  DETECTOR 

561.  In  the  Electrolytic  Detector,  use  is  made  of  the 
depolarising  effect  which  oscillatory  currents  produce 
on  a  polarised  electrolytic  cell. 

As  some  readers  may  be  unacquainted  with  simple 
chemical  theory,  we  think  it  advisable  first  of  all  to 
give  a  short  account  of  the  composition  of  water. 

COMPOSITION  OF  WATER 

562.  Water  is  described  chemically  by  the  formula 
H20,  which  means  that  it  consists  of  two  parts  by 
volume  of  the  gas  hydrogen  (H),  and  one  part  of  the  gas 
oxygen  (0),  in  chemical  combination  with  one  another. 

563.  Most  readers  will  have  noticed  that  if  a  gas-jet 
is  burned  in  a  small  room  for  any  length  of  time,  a 
considerable   amount   of   water   is   condensed   on   the 
wmdow  panes.     This  is  really  due  to  the  fact  that  the 


152  WIRELESS  TELEGRAPHY 

oxygen  in  the  surrounding  atmosphere  combines  chemic- 
ally with  the  hydrogen  in  the  coal  gas,  and  forms 
water.  This  chemical  action  produces  energy  in  the 
form  of  heat  which  is  apparent  from  the  very  high 
temperature  of  the  gas  flame. 

On  account  of  this  high  temperature,  the  water 
which  results  is  produced  hi  the  form  of  steam,  and  is 
therefore  invisible  until  it  has  had  time  to  condense  on 
to  some  body  which  will  conduct  the  heat  away  from  it. 

564.'  Hydrogen  and  oxygen  will  only  combine  when 
brought  to  a  high  temperature,  so  that  if  they  be  simply 
mixed  together  at  a  normal  temperature  no  chemical 
action  will  take  place,  but  as  soon  as  a  spark  or  flame 
is  applied  to  the  mixture  the  gases  immediately  sur- 
rounding the  spark  are  brought  to  a  high  temperature, 
with  the  result  that  chemical  action  is  started.  The 
heat  produced  by  this  action  is  more  than  sufficient  to 
keep  the  process  of  combustion  going,  so  that  even  if 
the  spark  or  flame  which  started  the  action  be  removed, 
the  gases  continue  to  combine  until  one  or  the  other 
of  them  is  exhausted. 

565.  Hydrogen  and  oxygen,  like  every  other  sub- 
stance, consist  of  a  very  large  number  of  minute  units 
to  which  scientists  have  given  the  general  name  of 
"  molecules."  Every  molecule  is  still  further  sub- 
divided into  units,  known  as  "  atoms,"  each  molecule 
consisting  of  a  definite  number  of  atoms.  The  molecule 
of  oxygen,  for  instance,  contains  two  atoms  of  oxygen ; 
the  molecule  of  hydrogen  contains  two  atoms  of  hydrogen; 
the  molecule  of  water,  however,  contains  three  atoms, 
namely,  two  of  hydrogen  and  one  of  oxygen. 

The  minuteness  of  a  molecule  can  be  gauged  from  the 
fact  that  the  smallest  particle  of  water  which  onn  bee. 


WIRELESS  TELEGRAPHY 


153 


seen  under  the  most  powerful  microscope  is  made  up 
of  many  millions  of  molecules. 

566.  Figs.  87  and  88,  which  show  a  simple  graphical 
illustration  of  the  molecular  theory,  will  enable  the 
reader  to  imagine  more  easily  the  difference  between  a 
mixture  and  a  combination.  In  these  illustrations  we 
have  represented  molecules  by  a  number  of  irregular 
outlines,  and  atoms  by  small  spheres  contained  inside 
the  molecule.  For  the  purpose  of  distinguishing  them 
we  have  represented  the  hydrogen  atoms  as  white  dots, 
thus,  o  ;  and  the  oxygen  atoms  as  black  dots,  thus, 


FIG.  87. 


Fro.  88. 


567.  Fig.  87  represents  a  number  of  hydrogen  and 
oxygen  molecules  mixed  together,  and  as  no  chemical 
action  is  yet  supposed  to  have  taken  place,  the  hydrogen 
and  oxygen  molecules  still  retain  their  individuality. 
If  this  mixture  be  ignited  the  result  of  the  chemical 
combination  of  the  hydrogen  and  oxygen  is  shown  in 
Fig.  88,  where  the  hydrogen  and  oxygen  no  longer  exist 
as  separate  molecules,  but  have  combined  in  the  pro- 
portion of  two  molecules  of  hydrogen  to  one  of  oxygen, 
and  form  entirely  new  molecules,  namely,  molecules 
of  water. 


154  WIRELESS  TELEGRAPHY 

This  brief  description  will  perhaps  assist  the  ivader 
to  understand  the  action  of  an  electrolytic  cell. 

ELECTROLYTIC  CELL 

568.  An  Electrolytic  Cell  is  an  apparatus  for  splitting 
up  the  molecules  of  water,  or  other  compounds,  into 
their  original  elements.     It  is  found  that  when  a  current 
of  electricity  is  made  to  pass  through  a  conducting 
solution  of  water,  the  water  is  decomposed,  that  is  to 
say,  the  molecule  of  water  is  split  up  into  the  three 
atoms,  two  of  which  are  hydrogen  and  one  oxygen. 

569.  An    electrolytic    cell,    therefore,    consists   of   a 
vessel  usually  of  glass,  containing  a  solution  of  sulphuric 
or  nitric  acid  in  water,  into  which  are  dipped  two  con- 
ducting  rods   ca'lled   the   electrodes.     These  rods   are 
usually  made  of  platinum  or  some  other  non-corrosive 
metal. 

570.  The  object  of  the  acid  is  to  make  the  water 
conducting,  pure  water   being  practically   a   non-con- 
ductor of  electricity. 

571.  Fig.  89  shows  such  an  electrolytic  cell  connected 
to  a  battery  as  the  source  of  E.M.F. 

The  electrode  to  which  the  positive  side  of  the  battery 
is  connected  is  called  the  anode,  and  the  electrode  to 
which  the  negative  side  of  the  battery  is  connected  is 
called  the  cathode. 

572.  The   current   flowing   through   the   electrolytic 
cell  from  the  battery  decomposes  the  water  at  a  rate  in 
proportion  to  the  strength  of  the  current  flowing  through 
the   cell.    .The   gases   thus   formed   are   not   liberated 
evenly  throughout  the  liquid,  but  collect  at  tlie  electrodes 
and  then  rise  to  the  surface  in  bubbles,  the  oxygen 
collecting  at  the  anode  and  the  hydrogen  at  the  cathode. 


WIRELESS  TELEGRAPHY  155 

As  water  consists  of  two  atoms  of  hydrogen  and  one 
of  oxygen,  twice  as  much  gas  will  collect  at  the  cathode 
as  at  the  anode. 

573.  The  amount  of  water  decomposed,  and  therefore 
tlie  amount  of  gas  given  off  at  the  electrodes,  is  pro- 
portional to  the  current  passing  through  the  liquid,  it 
follows,  therefore,  that  if  the  resistance  of  the  electro- 
lytic cell,  as  well  as  the  resistance  of  the  outside  circuit 
supplying  the  E.M.F.  necessary  to  pass  the  current 


FIG.  89. 

through  the  cell,  remained  constant,  the  amount  of  gas 
given  ofE  would  be  proportional  to  the '  voltage  applied 
across  the  cell. 

574.  This  condition,  however,  is  not  obtained  in 
practice,  for  when  only  a  small  voltage  is  applied  across 
the  cell  the  gas  is  not  produced  in  sufficient  quantity 
to  break  away  from  the  electrodes,  instead  it  sticks  to 
the  surface  of  the  platinum  in  the  form  of  tiny  bubbles, 
thus  reducing  the  area  of  contact  between  the  ele'ctrode 
and  liquid.  This  has  the  result  of  very  considerably 
increasing  the  resistance  of  the  electrolytic  cell.  This 
effect  is  known  as  polarisation,  and  when  the  gas  is 
removed  either  by  mechanical,  chemist  or  other  means, 
the  effect  is  known  as  de-polarisation. 


156  WIRELESS  TELEGRAPHY 

1>75.  In  the  case  of  electrodes  being  of  a  considerable 
size,  that  is  to  say,  having  a  large  surface  in  contact 
with  the  liquid,  the  increase  in  the  resistance  of  the  cell 
is  not  so  marked,  and,  moreover,  a  stronger  mechanical 
or  chemical  action  is  required  to  de-polarise  it  than  if 
the  contact  surface  between  the  electrode  and  liquid 
;is  very  small. 

576.  Now,  provided  the  electrode  has  a  sufficiently 
(small  area  of  contact  with  the  liquid  to  begin  with,  this 
polarising  action  continues  until  the  resistance  of  the 
cell  is  increased  to  such  an  extent  that  practically  no 

j  more  current  can  pass  through  it  at  that  voltage. 

577.  If  the  voltage  across  the  cell  be  then  increased, 
current  will  again  be  forced  through  the  cell  with  a 
corresponding  increase  in  the  amount  of  gas  formed, 
until  finally  the  gas  will  collect  in  sufficiently  large 
bubbles  to  break  away  from  the  electrodes  and  rise  to 
the   surface   of  the  liquid.    When  this   happens   the 
liquid  again  comes  into  contact  with  the  electrode,  with 
the  result  that  the  resistance  of  the  cell  drops ;  this 
effect  in  turn  allows  a  larger  current  to  flow  through 
the  cell,  and  the  gas  then  continues  to  be  liberated  in 
sufficient  volumes  to  bubble  freely  away  from  the  electrode 
until  the  current  is  cut  off. 

The  electrolytic  cell  is  most  sensitive  to  the  action 
I  of  the  high  frequency  currents  when  it  is  hi  the 
first  stage  of  the  process  described  above,  that  is,  when 
\the  initial  voltage  applied  across  the  cell  is  only  sufficient 
to  form  a  cushion  of  gas  round  the  electrode.  If  at  this 
stage  a  high-frequency  voltage  is  applied  to  the  cell, 
the  cushion  of  gas  is  apparently  broken  up  and  the 
liquid  again  comes  into  contact  with  the  electrode, 
causing  a  sharp  drop  in  the  resistance  of  the  cell.  Thus 


WIRELESS  TELEGRAPHY 


157 


we  may  say  that  the  action  of  high  frequency  oscillating 
currents  on  a  polarised  electrolytic  cell  is  to  de-polarise 
the  cell. 

578.  Figs.  90  and  91  show  how  the  electrolytic  de- 
tector can  be  connected  up  to  the  receiver  circuits.  If 
these  diagrams  be  compared  with  Figs.  75  and  77,  which 
show  the  method  of  connecting  a  crystal  detector  of 
the  receiver  circuits,  it  will  be  noticed  that  identically 
the  same  circuits  are  used  in  both  cases. 


T 


FIG.  91. 


579.  The  action  to  the  two   detectors,  however,  is 
fundamentally  different,  and  it  is  essential  that  this 
difference  be  thoroughly  understood. 

In  paragraphs  429-433  we  explained  that  the  function 
of  a  crystal  is  to  rectify  oscillating  currents  received, 
thus  converting  them  into  uni-directional  currents 
which  can  be  made  to  produce  audible  sounds  in  the 
ear-piece  of  a  telephone  (vide  also  paragraph  525) 

580.  Now  let  us  see  what  happens  in  the  case  of  an 
electrolytic    detector ;    by    following    the    diagram    in 
Fig.  90  it  will  be  seen  that  the  initial  voltage  across 
the  detector  D  is  provided  from  the  battery   B  and 
potentiometer  P.     By  means  of  the  potentiometer  the 


158  WIRELESS  TELEGRAPHY 

voltage  is  adjusted  to  such  a  value  that  the  resistance 
of  the  cell  is  at  its  highest  (vide  paragraphs  576  and  577). 
Since  the  telephones  T  are  in  series  with  the  detector, 
the  current  passing  through  the  telephones  will  then  be 
practically  zero.  In  any  case  what  little  current  there 
is  will  be  of  constant  value,  and  will  therefore  produce 
no  sound  in  the  telephones  (vide  paragraph  486).  As 
soon  as  a  train  of  high  frequency  oscillations  are  induced 
in  the  jigger  secondary,  the  electrolytic  detector  becomes 
de-polarised,  and  its  resistance  therefore  drops,  allowing 
an  increased  current  to  flow  from  the  battery  through 
the  telephones  and  through  the  detector,  this  increase 
in  the  current  producing  an  audible  click  in  the  tele- 
phones. 

581.  If  the  electrolytic  cell  is  suitably  made  it  will 
recover  its  polarisation  in  time  for  the  next  group  of 
oscillations,    so   that   each   group   of   oscillations   will 
produce  a  click  in  the  telephones ;   therefore,  as  in  the 
case  of  the  carborundum  detector,  the  sound  produced 
in  the  telephones  will  correspond  exactly  to  the  spark 
frequency  of  the  transmitter. 

582.  In  order  to  produce  an  electrolytic  detector 
which  will  in  the  first  place  be  sensitive  to  very  feeble 
oscillations,  and  also  which  will  recover  its  polarisation 
rapidly  enough  for  the  highest  practicable  spark  fre- 
quency, the  active  electrode  which  is  the  anode  must 
be  of  extremely  small  dimensions.     To  this  end  the 
anode  usually  consists  of  an  extremely  fine  platinum 
wire  fused  into  a  glass  holder,  a  magnified  sketch  of 
which  is  shown  in  Fig.  92.     The  end  of  the  platinum 
wire  projects  only  a  fraction  of  a  millimetre  beyond  the 
glass ;  thus  no  matter  how  much  the  glass  is  immersed 
in  the  electrolyte,  only  that  part  of  the  platinum  wire 


WIRELESS  TELEGRAPHY 


150 


which  projects  beyond  the  glass  will  come  into  contact 
with  the  liquid. 

583.  So  long  as  the  anode  is  small  the  size  of  the 
H 


Fio.  92. 

cathode  is  of  no  importance,  and  for  this  reason 
usually  consists  of  a  comparatively  thick  platinum  wire 
thoroughly  immersed  in  the  liquid  at  any  convenient 
distance. 

THE  MAGNETIC  DETECTOR 

584.  In  the  Magnetic  Detector  use  is  made  of  that 
property  in  iron  known  as  magnetic  hysteresis. 

MAGNETIC  HYSTERESIS 

If  a  piece  of  iron  be  brought  near  the  pole  of  a 
magnet,  that  part  which  is  nearest  the  magnet  becomes 
magnetised  to  an  opposite  polarity  by  magnetic  induc- 
tion (see  paragraph  83). 

585.  If  it  is  then  removed  from  this  magnetising 
force,  it  will  still  retain  a  certain  amount  of  the  magnetism 
induced  into  it,  by  reason  of  its  "  retentivity. 

586.  In  the  case  of  soft  iron  this  residual  magnetism 
is  extremely  unstable,   and  a  very  small  mechanical 
shock  or  twist  is  quite  sufficient  to  destroy  it. 

In- other  words,  the  magnetism  in  the  iron  does  not 


160  WIRELESS  TELEGRAPHY 

follow  exactly  any  change  in  the  magnetising  force,  but 
lags  a  little  behind  it  This  lagging  behind  is  called 
"  Hysteresis." 

587.  In  paragraph  98,  under  the  heading  of  "  Magnetic 
Induction,"' we  explained  how  an  electric  current  could 
be  induced  in  a  coil  of  wire  by  causing  a  change  in  the 
strength  of  the  magnetic  field  passing  through  the  coil. 

If,  then,  we  wind  a  coil  of  wire  round  an  iron 
core,  and  after  magnetising  the  latter,  we  subject  it  to 
a  mechanical  shock,  sufficient  to  destroy  its  residual 
magnetism,  a  current  will  be  induced  in  the  coil  of  wire. 
Moreover,  if  we  connect  a  pair  of  telephones  across  the 
coil,  thus  causing  the  current  induced  into  it  to  pass 
through  the  telephones,  we  shall  get  a  click  in  the 
telephones  when  the  iron  is  de-magnetised. 

Having  de-magnetised  the  iron  it  must  be  re- 
magnetised  before  a  similar  mechanical  shock  will 
produce  another  current  impulse  in  the  coil. 

588.  It  is  obvious  that  the  intensity  of  the  current 
induced  in  the  coil  of  wire  will  depend  on  the  difference 
between  the  amount  of  magnetism  in  the  iron  before  and 
after  it  is  subjected  to  the  de-magnetising  influence  of 
the  mechanical  shock. 

589.  A   very    feeble   shock    will   only   partially   de- 
magnetise the  iron,  with  the  result  that  a  feeble  sound 
is  produced  -in  the  telephones  ;  but  if  the  shock  is  suf- 
ficiently strong  to  destroy  all  the  magnetism  in  the  iron, 
we  shall  get  a  maximum  sound  in  the  telephones,  and 
any  further  increase  in  the  strength  of  the  shock  cannot 
further  increase  the  sound  produced  in  the  telephones. 

590.  We   have,   however,   a   means  of  still  further 
increasing  the  strength  of  the  current  induced  in  the  coil. 
Owing  to  its  hysteresis  the  iron  will  retain  its  residual 


WIRELESS  TELEGRAPHY  161 

magnetism,  not  only  when  the  magnetising  force  has 
been  removed,  but  also  when  it  has  been  reversed, 
provided  that  this  reversed  magnetising  force  is  not  too 
powerful. 

In  this  case  the  effect  of  our  subjecting  the  iron  to 
a  mechanical  shock  is  not  merely  to  destroy  its  residual 
magnetism,  but  to  allow  it  to  become  magnetised  in 
the  opposite  direction  by  the  influence  of  the  reversed 
magnetising  force. 

The  intensity  of  the  current  induced  in  the  coils  will 
then  be  proportional  to  the  amount  of  residual  magnetism 
in  the  iron  which  is  destroyed,  plus  the  amount  by  which 
it  is  magnetised  in  the  opposite  direction. 

591.  It  was   discovered    that   if    a   high   frequency 
oscillating  current  were  passed  through  a  coil  of  wire 
round  a  piece  of  iron,  it  produced  an  effect  on  the  iron 
similar  to  that  produced  by  a  mechanical  shock. 

592.  Let  us  now  see  how  these  principles  are  applied 
in  the  magnetic  detector. 

An  endless  band  B,  Fig.  93,  consisting  of  a  number 
of  fine  strands  of  iron  wire,  is  passed  over  two  pulleys 
P,  Pj,  one  of  which  is  kept  slowly  rotating  by  means  of 
clockwork,  thus  keeping  the  band  continuously  moving 
in  the  direction  indicated  by  the  arrow.  The  band  is 
made  to  pass  through  a  small  glass  tube  C,  around  which 
is  wound  a  single  layer  of  insulated  copper  wire,  the  two 
ends  of  which  are  connected,  one  to  the  aerial  and  the 
other  to  the  earth. 

A  second  coil  of  wire  D,  consisting  of  a  very  much 
larger  number  of  turns  of  wire,  is  also  wound  around  the 
glass  tube,  and  across  this  coil  is  connected  a  pair  of 
telephones  T.  A  single  horse-shoe  magnet  M  is  placed 
in  a  position  similar  to  that  shown  in  Fig.  93,  'with  one 

M 


162 


WIRELESS  TELEGRAPHY 


of  its  poles  (in  this  case  the  north  pole)  close  to  the  band 
a  short  distance  away  from  the  windings,  the  other  pole 
a  little  distance  away  from  the  band  near  the  middle  of 
the  windings. 

593.  Let  us  now  watch  the  progress  of  a  particular 
portion  of  the  band*  while  it  travels  from  the  point  X 
to  the  point  Y. 

It  first  of  all  approaches  the  north  pole  of  the  magnet, 
and  thereby  becomes  magnetised  as  a  south  pole  by 


FIG.  .93. 


magnetic  induction.  As  it  proceeds  farther  on  its  course 
it  gets  farther  and  farther  away  from  the  magnetising 
influence  of  the  north  pole  of  the  permanent  magnet, 
but,  owing  to  its  hysteresis,  it  will  retain  a  certain  amount 
of  magnetism.  As  it  enters  the  glass  tube  it  commences 
to  come  under  the  weaker  influence  of  the  south  pole  of 
the  magnet,  which  is  tending  to  make  it  into  a  north  pole, 


WIRELESS  TELEGRAPHY  163 

but  unless  disturbed  it  will  retain  its  original  residual 
magnetism  as  a  south  pole. 

594.  When  an  oscillating  current  is  induced  in  the 
aerial,   this  current  will  pass  >  round  the  single  layer 
winding  on  the  glass  tube  and  allow  the  magnetism  in  the 
iron  to  be  reversed,  thus  causing  a  sudden  change  in  the 
magnetic   field,   and   thereby   inducing   a   momentary 
current  in  the  secondary  coil,  to  which  the  telephones 
are  connected. 

595.  If,  on  the  other  hand,  no  oscillations  are  received 
m  the  aerial,  the  iron  will  pass  through  the  primary  tube 
without  having  its  magnetic  polarity  suddenly  changed, 
and    therefore    no    sound    will    be    produced    in    the 
telephones. 

596.  It  will  be  seen,  then,  that  we  have  a  continuous 
supply   of   iron   inside   the   primary   tube    in    such  a 
condition    that    oscillating    currents    passing    through 
the  coil  of  wire  will  cause  it  to  change  its  polarity 
suddenly. 

597.  Experience  has  shown  that  the  magnetic  detector 
is  quite  the  most  reliable  and  robust  form  of  receiver 
which  has  yet  been  invented,  but  although  extremely 
sensitive,  it  is  not  as  sensitive  as  the  modern  crystal 
detectors.     Its  reliability,  however,  makes  it  a  valuable 
instrument  as  a  stand-by,  or  in  places  where  experienced 
operators  are  not  obtainable. 

598.  To  tune  up  the  magnetic  detector  to  any  desired 
wave-length,  an  adjustable  inductance  and  an  adjustable 
condenser  are  joined  in  series  with  the  de-magnetising 
or  primary  winding  of  the  detector. 

599.  No    tuning   is    required   for    the   secondary    or 
telephone   winding,   for   the  currents  induced    in    the 
secondary  are   not  oscillatory    so   that  normally    the 


164 


WIRELESS  TELEGRAPHY 


magnetic  detector  can  be  regarded  as  a  single-circuit 
receiver. 

600.  As  this  arrangement  does  not  give  particularly 
sharp  tuning,  an  instrument  was  designed,  known  as 
the  Multiple  Tuner,  through  which  the  oscillations  have 
to  pass  before,  reaching  the  primary  winding  of  the 
magnetic  detector. 

601.  The    multiple  tuner  consists   of   three  oscilla- 


FIG.  94. 


tory  circuits  acting  inductively  upon  each  other,  each 
of  which  is  adjustable  as  regards  its  wave-length.  A 
diagram  of  connections  of  a  simple  form  of  this  tuner 
is  shown  in  Fig.  94. 

As  will  be  seen  from  the  diagram,  there  are  three 
distinct  circuits,  namely,  the  "  aerial "  circuit,  the 
"  intermediate  "  circuit,  and  the  "  magnetic  detector  " 
circuit. 

Each  of  these  circuits  must  be  in  tune  with  the 
wave-length  it  is  desired  to  receive. 


WIRELESS  TELEGRAPHY  16P 

602.  The   aerial    circuit   consists    of   an    adjustable 
inductance  A,  an  inductive  winding  B,  and  an  adjustable 
condenser  C,  all  connected  in  series  with  one  another. 

The  inductive  winding  B  is  so  placed  that  any 
oscillations  in  it  induce  similar  oscillations  in  the  inter- 
mediate circuit,  which  consists  of  two  inductive  windings 
D  and  E,  connected  in  parallel  and  across  an  adjustable 
condenser  F. 

Any  oscillations  induced  in  the  winding  D  will  also 
flow  in  the  winding  E,  since  the  two  windings  are  in 
parallel,  and  the  wave-length  of  the  circuit  is  adjusted 
by  changing  the  capacity  of  the  condenser  F. 

603.  The  third  circuit — the  magnetic  detector  circuit 
— consists  of  an  inductive  winding  G,  and  an  adjustable 
condenser  H,  and  by  itself  is  incomplete,  but  it  is  com- 
pleted  by  connecting   in  series  with    it   the    primary 
winding  of  the  magnetic  detector  as  shown  by  J. 

The  telephones  are  connected  as  described  before, 
across  the  secondary  coil  of  the  magnetic  detector 

"  ATMOSPHERICS  " 

604.  Electric  disturbances  in  the  atmosphere  which 
affect   the    receiving   apparatus   of    wireless    telegraph 
stations  are  known  by  the  name  of  "  atmospherics." 

They  produce  in  the  telephones  noises  which,  if  strong 
enough,  will  drown  the  signals  being  received 

Where  small  aerials  are  being  used,  these  atmospherics 
are  not  usually  troublesome,  but  where  large  and  high 
aertals  are  employed,  if  measures  were  not  taken  to 
reduce  their  effect,  it  would  be  impossible,  sometimes  for 
days  together,  to  communicate  at  all. 

605.  The  difficulty  in  getting  rid  of  "  atmospherics  " 


166  WIRELESS  TELEGRAPHY 

is  that  they  have  no  particular  tune,  but  will  cause  the 
aerial  circuit  to  oscillate  to  its  own  natural  frequency, 

so  that,  no  matter  to  what  wave-length  the  circuit  is  ad- 
justed, "  atmospherics  "  are  still  induced  in  the  receiver. 

606.  One  method  of  avoiding  them  can  be  described 
briefly  as  follows  : 

Two  receiving  circuits  are  opposed  to  one  another  in 
such  a  way  that  if  equal  effects  are  produced  in  each 
circuit,  these  effects  are  neutralised,  and  therefore  produce 
no  sound  in  the  telephones. 

If  one  of  these  circuits  is  in  tune  with  the  wave-length 
being  received,  and  the  other  circuit  is  out  of  tune  with 
this  wave-length,  unequal  effects  will  be  produced  in  the 
two  circuits  by  the  waves,  and  the  signals  will  be  received 
in  the  ordinary  way. 

The  "  atmospherics,"  however,  as  already  stated,  will 
affect  both  circuits  equally,  so  that  the  effect  of  the 
"  atmospherics  "  is  neutralised,  and  they  will  produce  no 
sound  in  the  telephones. 

Although  the  principle  of  this  method  is  simple, 
the  application  of  it  to  different  receivers  is  extremely 
complicated,  and  for  this  reason  we  have  not  described 
the  arrangement  in  detail. 

607.  There  is  another  form  of  "  atmospheric  "  called 
"  static,"  which    is    extremely    troublesome   should    a 
condenser  be  connected  in  series  with  the  aerial  for  the 
purpose  of  tuning. 

The  atmospherics  continually  charge  up  this  con- 
denser until  either  the  condenser  is  broken  down  or  the 
charge  sparks  across  the  two  sides  of  the  condenser. 

608.  "  Statics,"  however,  can  very  easily  be  dealt 
with  by  connecting  a  coil  of  wire,  as  shown  by  S,  Fig.  94, 
from  the  aerial  side  of  the  condenser  to  earth,  which 


WIRELESS  TELEGRAPHY  167 

allows  the  current  to  pass  through  the  coil  of  wire  to 
earth  instead  of  charging  up  the  condenser. 

It  is,  however,  necessary  that  this  coil  be  highly 
inductive,  as  otherwise  not  only  would  the  current 
caused  by  the  "  atmospherics  "  pass  through  it,  but  also 
the  oscillating  currents,  thereby  interfering  with  the 
tuning  effect  of  the  condenser  C. 

609.  For  this  reason,  in  nearly  all  receivers  that  are 
provided  with  aerial  tuning  condensers,  a  coil  of  wire, 
known  as  an  "  inductive  shunt,"  is  connected  from  the 
"  aerial  "  side  of  the  condenser  to  earth. 


AERIALS 

610.  The  function  of  an  aerial,  as  we  have  already 
shown,  is  twofold. 

In  the  first  place  it  is  required  to  radiate  energy  in 
the  form  of  aether  waves  from  the  oscillating  currents 
flowing  in  it.  In  this  case  it  may  be  said  to  act  as  a 
radiator. 

In  the  second  place  it  has  to  pick  up  energy  in  the 
form  of  oscillating  currents  from  aether  waves  which 
cross  it.  In  this  case  it  may  be  said  to  act  as  an  ab- 
sorber.. It  is  found  that  any  oscillatory  circuit  which 
is  efficient  as  a  radiator  (vide  paragraph  289)  will  also 
act  efficiently  as  an  absorber,  and  for  this  reason  in 
nearly  every  case  the  same  aerial  is  used  both  for  the 
purpose  of  transmitting  and  receiving. 

SHAPE  OF  AN  AERIAL 

611.  The   shape    any    particular    aerial    takes    will 
depend  upon  many  practical  considerations. 


168  WIRELESS  TELEGRAPHY 

The  shape  of  an  aerial  can  be  roughly  classified  under 
one    of    four    headings,    namely,    "  Vertical "    aerials, 


UMBRELLA       AERIAL 


I 


A\V\VS 

T     AERIAL 


INVERTED    L'     OR     "T       AE.RIAU 


FIG.  95. 


"  Umbrella  "  aerials,  "  T  "  aerials,  and  inverted  "  L  " 
aerials.     Fig.  95  represents  an  example  of  each  of  these 


612.  For  very  small  portable  stations  the  u1    jrella 


WIRELESS  TELEGRAPHY  169 

aerial  is  found  to  be  convenient,  principally  because  it 
only  requires  one  mast  to  support  it,  and  the  aerial, 
instead  of  putting  a  side  stress  on  the  mast  as  in  the 
case  of  "  T  "  or  inverted  "  L  "  aerials,  can  be  made  to 
act  as  a  set  of  stays,  thus  assisting  to  support  the 
mast. 

613.  The  "  T  "  aerial,  on  the  other  hand,  is  used 
largely  on  board  ship,  where  in  nearly  every  case  two 
masts  are  available  and  the  wireless  cabin  is  usually 
amidships. 

614.  Sometimes,  however,  it  is  more  convenient  to  use 
the  inverted  "  L  "  aerial,  on  account  of  the  position  of 
the  wireless  cabin.     The  inverted  "  L  "  aerial  is  used 
very  largely  where  very  long  aerials  are  required,  such 
as  in  the  case  of  long-distance  stations.     It  is  also  used^ 
largely  for  military  stations  of  medium  size,  on  account  of 
the  fact  that  it  is  easy  to  erect  in  almost  any  position.    It 
can,  for  instance,  be  as  easily  erected  in  a  road  or  street 
as  it  can  in  an  open  field ;    thus  for  military  purposes 
offering  a  great  advantage  over  the  umbrella  aerial. 

•SIZE  OF  AN  AERIAL 

615.  The  next  point  to  consider  is  the  size  of  an  aerial. 
This  depends  chiefly  upon  the  wave-length  it  is  required 
to  transmit,  which  in  turn  depends  to  a  large  extent 
upon  the  power  that  it  is  necessary  to  use. 

Every  aerial  has  its  own  natural  wave-length,  called 
its  fundamental  wave-length,  depending  upon  its  own 
capacity  and  its  own  inductance  (vide  paragraph  292 
onwards). 

616.  If  we  increase  the  length  of  an  aerial  we  in- 
crei    5  both  its  capacity  and  its  inductance,  and  thereby 


170  WIRELESS  TELEGRAPHY 

increase  its  fundamental  wave-length.  If  we  add  on 
to  an  aerial  another  parallel  wire,  we  increase  the 
capacity  of  the  aerial,  because  two  capacities  in  parallel 
result  in  a  larger  capacity,  but  at  the  same  time  we 
decrease  the  inductance  of  the  aerial,  because  two  in- 
ductances in  parallel  result  in  a  lower  total  inductance. 
Thus  it  is  found  that  by  adding  another  wire  to  an  aerial 
its  fundamental  wave-length  remains  more  or  less  un- 
altered. 

617.  If,  .however,  instead  of  keeping  the  additional 
wire  or  wires  of  the  aerial  parallel  with  one  another  we 
separate  them  out  radially,  as  in  the  case  of  the  umbrella 
aerial,  and  if  the  extremities  of  the  radial  wires  approach 
the  earth,  as  is  usually  the  case  in  the  umbrella  aerial  and 
also  in  some  forms  of  the  "  T  "  aerial,  then  the  capacity 
of  the  aerial  as  a  whole  is  increased  more  rapidly  than 
the  inductance  is  decreased,  because  the  inductance  of 
the  down  lead  is  unaltered,  with  the  result  that  'the 
fundamental  wave-length  is  increased. 

618.  In  practice  it  is  found  that  with  single- wire,  aerials 
^r  parallel-wire  aerials,  whose  wires  run  either  vertically 
or  horizontally,  the  wave-length  is  usually  about  four  and 
a  quarter  times  the  length  of  the  aerial.     With  a  "  T  " 
aerial,  the  upper  portion  of  which  is  kept  horizontal,  the 
fundamental  wave-length  is  about  five  times  the  length 
of  the  aerial,  but  if  the  ends  of  the  wires  are  brought  down 
so  as  to  approach  the  earth,  the  wave-length  will  be  still 
further  increased  in  proportion  to  the  length  of  the  wire. 

619.  With  an  "  umbrella  "  aerial,  the  wave-length 
may  be  as  much  as  eight  times  the  length  of  the  aerial, 
according  to  the  number  of  radial  wires  forming  it  and 
the  height  of  their  ends  from  the  earth,  and  the  height 
of  the  mast. 


WIRELESS  TELEGRAPHY  171 

Thus,  it  will  be  seen  that  two  or  more  aerials,  both 
having  exactly  the  same  fundamental  wave-length,  can 
have  different  proportions  of  capacity  and  inductance. 

620.  We  have  already  shown,  in    paragraph    294, 
that  we  can  increase  the  wave-length  of  the  aerial  by 
connecting  an  inductance  in  series  with  it.     Further, 
we  can  decrease  the  wave-length  of  an  aerial  by  con- 
necting a  condenser  in  series  with  it. 

We  are,  however,  limited  by  practical  considerations 
in  the  extent  to  which  we  can  thus  increase  or  decrease 
the  wave-length  of  an  aerial  from  its  fundamental  value. 
The  reason  for  this  is  that  the  aerial  is  most  efficient 
as  a  radiator,  and  therefore  also  as  an  absorber,  when 
neither  inductance  nor  capacity  has  been  connected  in 
series  with  it  (vide  paragraph  295). 

621.  For  thir  reason  it  is  usual  to  design  an  aerial 
so  that  its  fundamental  wave-length  is  approximately 
that  to  which  it  is  to   be  used  for  transmitting  or 
receiving. 

It  is  found  in  practice  convenient  for  a  station  to 
transmit  on  only  a  limited  number  of  wave-lengths,  but 
it  is  essential  that  the  same  station  be  able  to  receive 
over  a  very  large  range  of  wave-lengths,  and  therefore 
it  is  usual  to  consider  only  the  transmitting  requirements 
when  designing  the  aerial. 

622.  The  construction  of  an  inductance  coil,  suitable 
for  doubling  the  wave-length  of  a  given  aerial,  is  far 
cheaper  and  also  more  efficient  than  a  condensed  suitable 
for  halving  the  fundamental  wave-length  of  an  aerial, 
and  for  this  reason  the  aerial  is  usually  designed  to  have 
a  fundamental  wave-length  equal  to,  or  rather  shorter 
than,  the  shortest  wave-length  it  is  required  to  transmit. 
There  are,  of  course,  exceptions  to  this  rule  where  special 


172  WIRELESS  TELEGRAPHY 

conditions  have  to  be  fulfilled.     For  the  purpose  of  this 
book  it  is  unnecessary  to  deal  with  them  here. 


HEIGHT  OF  AN  AERIAL 

623.  The  height  of  an  aerial  is  a  very  important  con- 
sideration, because  it  is  found  that  the  range  oJ  a  station 
of  a  given  power  is  directly  proportional  to  the  average 
height  of  the  aerial.     Thus,  if  we  double  the  average 
height   of   the   aerial   of   a   given   station,  we   double 
the  range  of  that  station  without  increasing  the  power 
we  have  to  radiate. 

624.  It  depends,  of  course,  entirely  on  circumstances 
whether  it  is  cheaper,  or  for  other  reasons  more  con- 
venient, to  increase  the  height  of  an  aerial  or  to  increase 
the  power  of  the  station  in  order  to  increase  the  range. 
For  portable  stations  it  is  obviously  convenient  to  keep 
the  masts  as  low  as  possible,  and  to  keep  the  aerial  as 
simple  as  possible,  for  tall  masts  are  not  only  heavy  for 
carrying  about,  but  take  a  considerable  length  of  time 
to  erect. 

It  is.  found  in  practice  that  for  stations  that  are 
going  to  be  carried  about  by  hand  or  on  horseback, 
30  feet  is  a  very  convenient  height  of  mast,  although, 
where  time  taken  to  erect  is  not  of  primary  importance, 
masts  50  feet  or  even  70  feet  high  can  be  conveniently 
used. 

625.  Further,  the  cost  of  a  mast  very  rapidly  increases 
with  its  height,  and  it  therefore  becomes  a  question 
on  this  account  whether  it  is  cheaper  to  increase  the 
power  of  the  station  or  to  increase  the  height  of  the 
masts. 


WIRELESS  TELEGRAPHY 


173 


THE  ADVANTAGE  OF  USING  AERIALS  OF  A  LARGE 
CAPACITY 

626.  The  advantage  of  having  a  greater  capacity  iu 
the  aerial  is  very  apparent  when  we  try  to  increase 
the   wave-length   by   adding  an  inductance  in  series 
with   it.    Adding  an  inductance  to  an  aerial  reduces 
its  efficiency,  and  also  introduces  difficulties  of  insulation 
which  will  be  better  appreciated  after  reading  para- 
graph 632  onwards,  so  the  less  inductance  we  have  to 
add  to  obtain  the  required  wave-length  the  better. 

627.  A  large- capacity  aerial  requires  less  inductance  in 
series  with  it  to  increase  its  wave-length  to  a  given  value 


B 


\\\\\\\\\\\\\ 


Fia. 


than  a  small-capacity  aerial,  assuming,  of  course,  that 
the  fundamental  wave-lengths  of  the  two  aerials  are  the 
same. 


174  WIRELESS  TELEGRAPHY 

It  is  quite  easy  to  show  this  by  the  application  of 
the  formula  .given  in  paragraph  259,  namely  : 

Xm  =  1885  «/C(mf)  x  L(mh) . 

We  will  suppose  that  the  fundamental  wave-length 
of  each  of  the  aerials'  A  and  B,  shown  in  Fig.  96,  is 
100  metres,  but  in  the  aerial  B  the  capacity  is  larger, 
and  therefore  the  inductance  is  smaller  than  in  the 
aerial  A.  We  will  suppose  that  in  the  aerial  A 

C=-0002  microfarad 

L  =  14- 1  microhenries, 
and  in  the  Aerial  B 

C=-0004  microfarad 

L  =  7-  05  microhenries ; 
then,  without  any  extra  inductance, 

Xm  of  aerial  A  =  1885  N/-0002  x  14'1 

=1885  x- 053 
=100  metres, 
and  also 

Xm  of  aerial  B  =  1885  */'0004  x  7 '05 
=  1885  x -053 
=  100  metres. 

Now  let  us  add  on  to  each  aerial  an  additional  in- 
ductance of  10  microhenries.  Then  we  shall  have— 

Xm  of  aerial  A  =  1885  N/'0002x24-l 
=18&5x-0695 
=131  metres  (about) ; 


WIRELESS  TELEGRAPHY  175 

but 

\m  of  aerial  B  =  1885<v/'0004  x  17'05 
=  1885x0-826 
=156  metres  (nearly). 

628.  It  will  be  seen  that  with  the  same  additional 
inductance  we  have  increased  the  wave-length  of  the 
aerial  B  from  100  metres  to  156  metres,  while  we  have 
only  increased  the  wave-length  of  the  aerial  A  from 
100  metres  to  131  metres. 

Thus,  if  we  wished  to  increase  the  wave-length  of 
the  aerials  shown  in  Fig.  96  from  425  feet  to,  say,  600 
feet,  we  should  find  that  perhaps  10  turns  of  an  in- 
ductance coil  would  be  required  in  the  case  of  the 
aerial  A,  while  only  about  six  turns  would  be  required 
in  the  case  of  the  aerial  B,  as  illustrated. 

THE  LENGTH  OP  AN  AERIAL 

629.  The  length  of  an  aerial  is  not  necessarily  the 
total  length  of  wire,  but  is  the  length  of  wire  from  the 


Fio.  97. 


point  where  it  is  connected  to  the  instruments  to  any 
one  of  its  extremities.     Thus  in  the  "  umbrella  "  aerial 


176 


WIRELESS  TELEGRAPHY 


shown  in  Fig.  97  the  length  of  the  aerial  is  105  feet, 
made  up  of  30  feet  of  "  down-lead  "  and  75  feet  of  radial 
wires. 

630.  The  length  of  the  "  T  "  aerial  shown  in  Fig. 
98  is  150  feet,  made  up  by  50  feet  of  "  down-lead  "  and 
100  feet  of  horizontal  wire  in  a  200-foot  span. 


^v 


x/ 


FIG.  98. 


FIG.  99. 


631.  And  the  length  of  the  "  L  "  aerial  shown  in  Fig. 
99  is  200  feet,  made  up  by  50  feet  of  "  down-lead  " 
and  150  feet  of  horizontal  wire. 


WIRELESS  TELEGRAPHY 


177 


DISTRIBUTION  OF  POTENTIAL  AND 
CURRENT  ALONG  AERIALS 

632.  Before  going  into  the  question  of  the  construc- 
tion and  insulation  of  aerials,  let  us  first  consider  how 
the  voltage  of  the  aerial  is  distributed  when  oscillating 
currents  are  flowing  in  it. 

633.  Taking  the  case  of  an  aerial  directly  energised 
by  an  induction  coil,  i.e.  "  plain  aerial,"  the  maximum 
initial  voltage  to  which  the  aerial  is  charged  will  depend, 
as  we  have  shown,  upon  the  voltage  applied  acrosscthe 
spark-gap  by  the  induction  coil.     This  voltage  can  be 
regulated  by  increasing  or  decreasing  the  length  of  the 
spark-gap. 

634.  (The  voltage  required  to  jump  an  air-gap  where 
points  are  used  for  the  electrodes  is  about 

12,000  volts  for  each  centimetre  length  of  air- 
gap.  If  knobs  or  balls  are  used  for  elec- 
trodes, the  voltage  required  will  be  increased 
to  an  extent  depending  upon  the  curvature  of 
the  ball.  Thus,  using  balls  one  inch  in  dia- 
meter, it  is  found  that  a  voltage  of  approxi- 
mately 30,000  volts  for  each  centimetre 
length  of  air-gap  is  required.) 

635.  Assuming  for  the  sake  of  explanation    \ 30.000 
that  we  are  using  ball  electrodes  set  one  centi-    [^     **  C\ 
metre  apart,  then  at  the  instant  immediately 

before  the  gap  is  broken  down  the  whole  aerial 

is  charged  up  to  a  uniform  potential  of  30,000 

volts.     In  this  case  the  distribution  of  the 

voltage  along  the  aerial  wire  can  be  shown  diagram- 

matically,  as  in  Fig.  100,  by  a  dotted  line  drawn  parallel 


woo 


FIG.  100. 


178  WIRELESS  TELEGRAPHY 

with  the  aerial  at  a  distance  from  it  representing  30,000 
volts. 

636.  Now  the  energy  stored  up  in  a  condenser  can 
be  regarded  as  "  potential  "  energy,  as  opposed  to  the 
"  kinetic  "  energy  which  is  stored  up  in  an  inductance 
through  which  a  current  is  flowing ;  just  as  the.  mechani- 
cal energy  stored  up  in  a  compressed  spring  is  in  the 
form  of  "potential"  energy  as  opposed  to  the  "kinetic" 
energy  which  is  stored  up  in  a  revolving  fly-wheel  or 
in  any  moving  body. 

637.  In  all  periodic  oscillations,  whether  mechanical 
or.  electrical,' the  energy  is  continually  changing  from 
the  one  form  to  the  other.      Thus,  for  example,  in 
the  case  of  a  vibrator,  as  shown  in  Fig.  37,  which  is 
made  to  vibrate  between  the  positions  W:  and  W2,  when 
it  occupies  one  or  other  of  these  extreme  positions,  it 
is  for  the  moment  at  rest  and  all  the  energy  is  then 
in  the  form  of  "  potential  "  energy  stored  up  in  the 
tension  of  the  spring  blade.    On  the  other  hand,  when  it 
occupies  the  position  W,  there  is  for  the  moment  no 
tension  in  the  spring,  but  at  this  moment  the  weight 
W  is  travelling  at  its  maximum  speed,  consequently 
all  the  energy  is  then  in  the  form  of  "  kinetic  "  energy 
stored  up  in  the  moving  body  W      At  intermediate 
positions  obviously  part  of  the  energy  is  stored  up  in 
the  tension  of  the  spring  and  part  of  it  in  the  moving 
weight  W 

Similarly,  in  the  case  we  are  now  considering,  namely, 
that  of  an  oscillatory  circuit,  the  energy  at  one  instant  is 
all  stored  up  in  the  condenser  in  the  form  of  "  potential " 
energy,  arid  at  this  moment  the  electricity  is  at  rest, 
that  is  to  say,  there  is  no  current  flowing.  At  the  next 
instant  when  the  condenser  is  fully  discharged  and 


WIRELESS  TELEGRAPHY  179 

before  it  com  indices  to  be  charged  in  the  Opposite 
direction,  the  current  flowing  in  the  circuit  is  at 
its  maximum,  and  all  the  energy  is  then  in  the  form 
of  "  kinetic  "  energy  stored  up  in  the  current  flowing 
through  the  inductance  of  the  circuit.  Thus  it  will  be 
seen  that  the  total  energy  in  an  aerial  or  other  oscillatory 
circuit  at  any  moment  is  the  sum  of  the  "  potential  " 
energy  and  the  "  kinetic  "  energy. 

638..  In  paragraph  285,  we  showed  how  the  energy 
stored  up  in  a  condenser  at  any  moment  could  be 
determined  from  the  equation  E=|CV2,  where  C  is 
the  capacity  of  the  condenser ;  and  V  the  E.M.F.  to 
which  it  is  charged. 

Taking  the  analogous  mechanical  case  of  energy 
stored  up  in  a  compressed  spring,  it  can  be  shown  that 
at  any  moment  the  energy  E  =|(F)T2,  where  (F)  is  the 
flexibility  l  of  the  spring,  and  T  the  tension  to  which 
it  is  stressed. 

639.  Now  the  energy  stored  up  in  a  moving  body, 
i.e.  the  "  kinetic  "  energy,  depends  upon  the  weight  of 
that  body  and  the  speed  at  which  it  is  travelling.     The 
energy  is  directly  proportional  to  the  weight  of  the 
moving  body  and  proportional  to  the  square  of  the  speed 
at  which  it  is  travelling.     If  M  is  taken  to  represent  the 
mass  of  the  body  and  V  to  represent  its  velocity,  then 
it  can  be  shown  that  energy  stored  up  in  it  at  any 
moment  E  =|MV2. 

640.  Similarly,  in  an  electrical   circuit  the  energy 
stored  up  in  the  inductance  L  of  the  circuit  through 

1  Note  the  term  flexibility  is  used  for  the  sake  of  simplicity, 
and  the  equation  will  be  found  correct  if  F  is  taken  equal  to  ^, 

where  L  is  the  elongation  of  the  spring,  and  E  is  the  modulus  of 
elasticity. 


180  WIRELESS  TELEGRAPHY 

which  a  current  is  flowing,  is  directly  proportional  to 
the  inductance  and  proportional  to  the  square  of  the 
current  flowing  through  that  inductance.  If  J  =  energy 
in  joules,  L  ^inductance  in  henries,  and  I  =  current  in 
amperes,  it  can  be  shown  that  J  = JLI2 

641.  Returning  to.  the  particular  case  we  are  consider- 
ing, namely,  that  of  an  aerial  which  is  uniformly  charged 
to  a  pressure  of  30,000  volts,  then  it  is  evident- that  at 
the  moment  before  the  spark-gap  is  broken  down,  all 
the  energy  is  stored  as  "  potential "  energy,  because  at  this 
moment  there  is  no  current  flowing ;    therefore  if  C  is 
the  capacity  of  the  aerial,  and  V  the  voltage  to  which 
it  is  charged,  J  =|t!V2. 

At  the  next  instant  the  gap  is  broken  down,  and 
the  "  potential  "  energy  in  the  charged  aerial  is  gradually 
transferred  to  "  kinetic  "  energy  in  the  form  of  a  current 
of  electricity  passing  through  the  inductance  of  the  aerial 
to  earth. 

642.  If  L  represents  the  inductance  of  the  aerial,  and 
I  represents  the  current  flowing  through  that  inductance, 
and  J  again  represents  the  energy  in  the  aerial,  then, 
when  the  current  is  oscillating,  the  energy  in  the  aerial 
at  any  instant  =|C(V)2  +  |L(I)2. 

643.  Taking  the   instant  when   one-quarter   of   an 
oscillation  has  taken  place,  the  Voltage  of  the  aerial  has 
become  zero,  and  therefore  £CV2=0.    At  this  instant, 
therefore,  the  whole  of  the  energy  is  transferred  to  the 
current  flowing  through  the  inductance  and  will  then 
equal  |LI2,  and  consequently  the  current  in  the  aerial 
is  at  its  maximum. 

Similarly,  taking  the  instant  when  one-half  oscillation 
has. taken  place,  the  current  Jhas  become  zero,  and 
therefore  at  this  instant  the  whole  of  the  energy  is 


WIRELESS  TELEGRAPHY  181 

transferred  to  the  charge  in  the  capacity  of  the  aerial, 
and  will  then  equal  £CV2,  and  consequently  the  voltage 
of  the  aerial  is  at  its  maximum. 

644.  Assuming,  for  the  moment,  that  none  of  the 
energy  is  lost  either  in  radiation  or  resistance,  then  the 
energy  in  the  aerial  will  be  the  same  at  the  end  of  the  first 
oscillation  as  it  was  originally  at  the  moment  immediately 
before  a  spark  occurred,  but  since  the  spark-gap  is  now 
broken  down  the  bottom  end  of  the  aerial 

must  be  considered  as  connected  to  earth,    j-   . 
and  therefore  the  voltage  at  this  point    \ 
will  remain  at  zero  while  the  maximum     \ 
voltage  will  be  found  at  the  free  end  of      \ 
the  aerial.    Thus,  it  will  be  seen  that  the       \ 
distribution    of    voltage  over  the  length        \ 
of  the  aerial  will  take  a  different  form         \ 
from  that  shown  in  Fig.  100.     It  will  take          \ 
the  form  of  the  curve  shown  in  Fig.  101.  \ 

At  first  sight  it  would  appear  that  the  \ 

maximum  value  of  the  voltage  obtained  v^    - 

at  the  free  end  of   the  aerial  would  be    0  VoLTS 
the   voltage   to   which  it  was  originally 
charged.    This,  however,  is  not  the  case,        FIG.  101. 
for  the  following  reason. 

645.  At  the  moment  when  the  aerial  is  first  charged, 
the  charge  is  uniformly  distributed  over  the  whole  of 
the  aerial,  as  shown  in  Fig.  100,  and  since  the  capacity  of 
the  aerial  is  also  distributed  over  the  whole  of  the  aerial, 
it  follows  that  the  whole  of  the  capacity  of  the  aerial  is 
charged  to  an  equal  voltage. 

646.  On  the  other  hand,  after  the  first  oscillation 
when  the  charge  in  the  aerial  is  distributed,  as  shown 
in  Fig.   101,  the  whole  of  the  capacity  in  the  aerial 


182 


WIEELESS  TELEGRAPHY 


is  not  charged  to  the  same  voltage,  and  therefore  the 
free  end  of  the  aerial  must  necessarily  become  charged 
to  a  higher  voltage  than  originally,  in  order  that  the 
aerial  may  store  up  the  same  amount  of  energy  as 
before. 

647.  It  can  be  shown  mathematically  th'at  the.  voltage 
at  the  free  end  of  the  aerial  at  the  end  of  a  complete 
oscillation  will  be  -\/2  times,  or  approximately  1-414  times 
the  voltage  to  which  it  was  originally  charged,  assuming 
(1)  that  the  voltage  is  then  distributed  in  the  form  of  a 
sine  curve,  and  (2)  that  no  energy  has 
been  lost  or  radiated  during  the  oscilla- 
tion. This  will  be  readily  understood  by 
referring  to  Figs.  102  and  103. 

648.  Fig.  102  shows  the  relative  distri- 
bution of  the  voltage  along  the  aerial  wire, 
the  dotted  line  showing  the  original  charge 
put  into  the  aerial  by  the  induction  coil, 
and  the  full  line  showing  the  charge  in  the 
aerial  at  the  end  of  the  first  oscillation. 

649.  Fig.  103  shows  the  variation  of 
voltage  at  the  free  end  of  the  aerial  wire 
during  the  first  oscillation.      Up  to  the 
point  A  the  curve  shows  the  compara- 
tively slow  rise  of  voltage,  while  the  aerial 

is  being  uniformly  charged  up  by  the  induction  coil  to 
a  value  of  30,000  volts.  At  the  point  A  the.  spark-gap 
breaks  down,  and  oscillations  commence,  and  the  voltage 
at  the  end  of  the  aerial  first  drops  to  zero  at  the  moment 
B  when  the  aerial  is  discharged  and  then  rises  to  a 
value  /v/2  times  the  original  charge  =  about  42,400 
volts. 

650.  We  have  said  that  the  energy  stored  up  in  the 


O  VOLTS 


FIG.  102. 


WIRELESS  TELEGRAPHY 


183 


aerial  when  the  whole  of  I  he  charge  is  in  the  aerial     \(  '  V2 
This  is  only  true  when  the  whole  of  the  capacity  of  the 

42.^00 
30000 


42.400       - 


FIG.  103. 


aerial  is  uniformly  charged  to  the  voltage  V.     When  the 
aerial  is  oscillating,  the  effective  capacity  of  the  aerial 


must  be  taken  as  ~ 


the  true  capacity. 


DISTRIBUTION  OF  CURRENT  IN  AN  AERIAL 

651.  We  have  already  explained  in  paragraph  643, 
that  at  the  moment  when  the  voltage  at  the  end  of  the 
aerial  is  at  its  maximum,  the  current  flowing  into  the 
aerial  is  at  zero,  and  the  distribution  of  the  voltage  along 
the  aerial  when  the  latter  is  oscillating  to  its  fundamental 
wave-length  is  such  that  there  is  a  node  of  potential 
where  the  aerial  is  connected  to  earth,  and  an  anti-node 
of  potential  at  the  free  end  of  the  aerial.  The  distribu- 
tion of  the  current  is  the  reverse  of  this. 

It  is  obvious  that  the  maximum  current  will  flow  at 
the  earth  end  of  the  aerial,  for  all  of  the  current  which 
flows  into  the  aerial  must  necessarily  pass  this  point, 


184 


WIRELESS  TELEGRAPHY 


whereas,  taking  a  point  half-way  up  the  aerial,  only  that 
current  which  is  required  to  charge  the  upper  half  of  the 
aerial  will  flow  past  this  point,  and  taking  the  extreme 
end  of  the  aerial  no  current  can  flow  through  it.  Thus  the 
distribution  of  the  current  in  an  aerial  will  also  take  the 
form  of  a  sine  curve,  but  with  its  anti-node  at  the  point 
where  the  aerial  is  connected  to  earth,  and  its  node  at 
the  free  end  of  the  aerial ;  for  this  reason  the  effective 

inductance  o!  the  aerial  must  be  taken  as 

2 


Zero  RMPS 


times  the  true  inductance. 

7T 

652.  We  may  draw  a  curve,  as  shown 
by  the  dotted  line  in  Fig.  104,  representing 
the  distribution  of  the  current  flowing 
along  an  aerial,  where  the  distance  between 
the  curve  and  the  full  line  representing 
the  aerial  wire  represents  the  compara- 
tive amount  of  current  flowing.  This 
current  will  vary  from  a  maximum  value 
when  the  potential  of  the  aerial  is  at 
zero,  to  zero  when  the  potential  of  the 
aerial  is  at  its  maximum,  but  the  distri- 
bution of  the  current  along  the  aerial  will 
be  always  in  the  same  proportion,  so  that  although  the 
amplitude  of  the  curve  will  vary,  the  form  of  the  curve 
will  remain  the  same. 


•a 


/Mnx. 

.h-   HMPS 


Fia.  104. 


INFECT  ON  CURRENT  AND  VOLTAGE  DISTRIBUTION  OF 
CONNECTING  AN  INDUCTANCE  OR  CAPACITY  IN 
SERIES  WITH  AN  AERIAL. 

653.  Jt  is  evident  that  whatever  form  the  distribu- 
tion of  tfie  voltage  along  an  aerial  takes,  the  distribution 


WIRELESS  TELEGRAPHY 


185 


of  tbo  current  will  always  take  a  relative  form,  but  with 
its  node  at  the  point  where  the  voltage  is  at  its  maximum, 
and  vice  versa,  with  its  anti-node  at  the  point  wJiere  there 
is  a  voltage  node.  In  describing  further  effects,  therefore, 
it  will  be  sufficient  only  to  indicate  the  distribution  of 
the  voltage  along  different  aerials. 

654.  When  an  inductance  is  connected  in  series  with 
an  aerial,  the  distribu- 
tion of  voltage  along 

the  aerial  when  oscil- 
lating is  similar  to  that 
already  described  for 
a  simple  aerial,  except 
that  the  inductance 
must  be  regarded  as 
a  continuation  of  the 
aerial,  and  therefore 
the  voltage  increases 
along  the  inductance 
as  well  as  along  the 
aerial,  as  shown  in 
Fig.  105;  thus  the 
greater  the  inductance 
that  is  connected  in 
series  with  an  aerial, 
the  higher  will  be  the 
voltage  across  that  inductance  when  it  is  oscillating. 

655.  It  is  most  important  to  bear  this  point  in  mind 
when  designing  inductance  coils  to  be  used  for  transmit- 
ting purposes,  for  the  coil  must  be  very  highly  insulated 
from  earth  at  the  end  which  is  connected  to  the  aerial, 
and,  further,  when  there  is  a  large  amount  of  inductance 
connected  in  series  with  the  aerial,  very  high  insulation 


Fio.  105. 


FIG.  106. 


186 


WIRELESS  TELEGRAPHY 


must    bo.   provided   where  the  aerial   wire   enters   the, 
building. 

656.  The  effect  on  the  distribution  of  the  voltage^  of 
connecting  a  condenser  in  series  with  an  aerial,  is  to 
create  a  node  of  potential  in  the  aerial  at  some  point 
above  the  condenser,  as  shown  in  Fig.  106. 

657.  The  exact  position  of  the  node  will  depend  upon 
the  relative  values  of  the  capacity  of  the  condenser,  and 
the  capacity  of  the  aerial. 

Taking  the  two  possible  extreme  values  of  capacity  of 
a  condenser,  we  find  that  if 
an  infinitely  large  capacity 
be  connected  in  series  with 
an  aerial,  the  node  will  be 
found  exactly  at  the  junc- 
tion of  the  aerial  and  the 
condenser,  as  shown  in  Fig. 
107,  for  an  infinitely  large 
capacity  is  equivalent  to 
a  direct  connection  to  earth. 
On  the  other  hand,  if  we 
connect  an  infinitely  small 
capacity  in  series  with  an 

aerial,  the  node  of  potential  wjll  occur  exactly  half-way 

up  the  aerial,  as  shown  in  Fig.  108. 

658.  Thus  any  intermediate  capacity  between  the 
values  of  infinity  and  0  will  create  a  node  somewhere 
between  the  bottom  of  the  aerial  and  half-way  up  the 
aerial,  according  to  the  relative  values  of  the  capacity 
of  the  aerial  and  the  capacity  of  the  condenser. 


1 

1 

1 

1 

1 

1 

1 

1 

1 

I 

1 

1 

1 

1 

1 

1 

CRPKITY 

l 

IN_ 

i 

lMr\ 

Fio.  107. 

Fio.  108. 


WIRELESS  TELEGRAPHY 


187 


HARMONICS 

659.  Any  oscillatory  circuit  in  which  the  capacity  and 
inductance  are  distributed,  that  is  to  say,  any  "  open  " 
oscillatory  circuit,  will  oscillate  to  harmonics  of  the 
fundamental  wave. 

The  first  harmonic  has  a  frequency  of  three  times  the 
fundamental  frequency,  the  second  harmonic  five  times, 
and  the  third  harmonic  seven  times,  and  so  on.  Thus 
the  wave-length  of  the  first  harmonic  will  be  one-third 
the  fundamental  wave-length,  that  of  the  second  har- 
monic one-fifth,  and  that  of  the  third  one-seventh. 

660.  When  the  aerial  is 
oscillating  to  the  first  har-    ' 
mom'c    the   distribution    of    \ 
the  voltage  along  it  will  take    \ 
the  form  shown  in  Fig.  110, 
from  which  it  will  be  seen 
that   there  are  two    points 
on  the  aerial,  of  maximum 
voltage,  one  at  the  end  of 
the  aerial   and  the  other  a 

third  of  the  way  up  the  aerial.  Further,  a  node  of 
voltage  is  obtained  at  a  point  two-thirds  of  the  way 
up  the  aerial,  as  well  as  at  the  point  where  the  aerial 
is  connected  to  earth. 

The  distribution  of  voltage  along  an  aerial  oscillating 
to  its  fundamental,  its  first  harmonic,  second  harmonic, 
and  third  harmonic,  is  shown  diagrammatically  in 
Figs.  109,  110,  111,  and  112  respectively. 

661.  These  harmonics  can  be  distinctly  detected  with 
a  sensitive  wavemeter  when  an  aerial  is  excited  as  "  plain 
aerial."     It  will  be  noticed,  however,  that  the  funda- 


\ 

\ 

\ 

\ 

)      f. 

}  / 

''       \ 

i    \^ 

F      ^ 
io.        F 

F       "^ 
[0.        F 

W      1 

IG.           Fl 

09.        110.        111.        11 

188  WIRELESS  TELEGRAPHY 

mental  wave-length  gives  by  far  the  strongest  effect  in 
the  wavemeter.  The  first  harmonic  will  be  very  much 
stronger  than  the  second,  and  "the  second  harmonic 
stronger  than  the  third,  and  so  on.  Difficulty  will  be 
found  in  detecting  any  of  the  higher  harmonics  on 
account  of  their  weakness. 

662.  When    an    aerial  is    directly   excited,  as,   for 
instance,  by  means  of  an  induction  coil,  the  harmonics 
are  only  feebly  produced,  and  when  an  aerial  is  excited 
indirectly  by  a   coupled   "  closed "   oscillatory  circuit 
which  is   tuned   to   the  fundamental   wave-length   of 
the  aerial,   the   harmonic   wave-lengths   of  the  aerial 
will  be  even  more  feebly  produced,  practically  all  of 
the  energy  being  radiated  in  the  fundamental  wave- 
length. 

663.  If,  however,  the  coupled  "  closed  "  oscillatory 
circuit  be  tuned  to  one  of  the  harmonics  of  the  aerial,  the 
aerial  will  not  oscillate  to  its  fundamental  wave-length, 
and   consequently   only  the  harmonic    to    which    the 
"  closed  "  oscillating  circuit  is  tuned  will  be  radiated.   As 
a  matter  of  fact,  an  aerial  excited  to  one  of  its  harmonics 
will  radiate  more  rapidly  than  when  excited  to  its  funda- 
mental wave-length.     Use  is  therefore  sometimes  made 
of  this  phenomenon  to  avoid  the  necessity  of  inserting 
a  condenser  or  a  large  inductance  in  series  with  the 
aerial,  thus  reducing  its  efficiency  as  a  radiator,  when 
a  station  is  required  to  transmit  a  long  wave-length  and 
a  short  wave-length  on  the  same  aerial,  and  where  it 
is  possible  to  arrange  that  the  short  wave-length  is  a 
harmonic  of  the  long  wave-length. 

664.  In  the  early  days  of   wireless  telegraphy,  all 
ships  fitted  with  wireless  could  transmit  on  either  of  two 
wave-lengths,  which  were  called  respectively  "  Tune  A  " 


WIRELESS  TELEGRAPHY  189 

and  "  Tune  B."  Tune  A  was  a  wave-length  of  3GO  feet, 
and  Tune  B  was  a  wave-length  of  1080  feet.  Thus  it 
was  usually  arranged  that  the  wave-length  of  the  aerial 
was  approximately  1080  feet,  and  that  the  primary 
circuit  of  Tune  A  was  tuned  to  the  first  harmonic  of  this 
aerial,  namely,  360  feet,  and  that  of  Tune  B  to  the 
fundamental. 

It  is,  however,  not  possible  to  arrange  this  now,  for 
the  International  Convention  of  Radio-Telegraphy  have 
laid  down  that  all  ships  must  be  able  to  transmit  wave- 
lengths of  either  600  metres  or  300  metres,  and  in.  this 
case  the  lower  wave-length  is  not  a  harmonic  of  the 
higher  wave-length. 


MASTS 

665.  In  paragraph  623,  we  pointed  out  that  the  range 
over  which  a  given  transmitter  can  communicate,  is 
approximately  proportional  to  the  height  of  the  aerial. 
It  is  obvious,  therefore,  that  where  range  of  communica- 
tion is  of  paramount  importance,  the  masts  used  for 
supporting  the  aerial  should  be  as  high  as  practical 
consideration  will  allow. 

The  masts  of  permanent  Land  Stations  are  usually 
erected  by  experienced  engineers  and  riggers,  and  skilled 
men  are  usually  available  for  keeping  the  masts  in  good 
repair,  but  the  masts  used  on  portable  stations  are 
frequently  handled  by  men  who  have  had  no  such 
experience,  and  we  therefore  think  that  a  few  remarks 
on  the  subject  will  be  useful. 


190  WIRELESS  TELEGRAPHY 

STRAIN  ON  MASTS 
t>66.  A   mast   will   withstand   a   very  much  greater 


FIG.  113. 


FIG.  114. 


stress  acting  straight  down  its  length  than  it  will  one 
acting  at  right  angles  to  its  length. 


FIG.  115. 


A  simple  experiment  with  a  stick  about  \  inch 
diameter  by  5  feet  long  will  readily  illustrate  this  point. 
If  we  exert  a  pull  of  say  10  Ib.  on  one  end  of  the  stick 


WIRELESS  TELEGRAPHY 


191 


in  a  direction  at  right  angles  to  its  length,  as  shown  in 
Fig.  113,  the  stick  will  probably  break,  or  at  all  events 
bend  very  sharply.  .If,  however,  we  exert  the  same 
force  on  the  end  of  the  stick  in  a  direction  in  line  with 
its  length,  as  shown  in  Fig.  114,  the  stick  will  withstand 
it  easily. 

667.  In  the  case  of  a  mast  supporting  an  umbrella 
aerial,  as  shown  in  Fig.  115,  the  result  of  all  the  forces 
exerted  by  the  wires  is  a  force  acting  straight  down  the 
length  of  the  mast,  and  therefore  the  best  advantage  is 
being  made  of  the  strength  of  the  mast. 

668.  But  in  the  case  of  a  mast  supporting  a  horizontal 
aerial,  as  shown  in  Fig.   ll6, 

where  the  aerial  is  attached 
to  the  top  of  the  mast,  the 
force  exerted  by  the  aerial  is 
at  right  angles  to  the  length  of 
the  mast,  and  therefore  the 
strength  of  the  mast  is  not 
being  used  to  the  best  advan-  MOST 


PULL  OF  PETRIPL 


669.  If,  however,  we  attach 
a  stay  to  the  top  of  the  mast, 
and  connect  the  stay  to  a 
point  on  the  ground  some  dis- 
tance from  the  foot  of  the  -^ 
mast,  as  shown  in  Fig.  117,  x 
the  pull  of  the  aerial  will  be 
carried  by  the  pull  of  the  stay,  and  the  resultant  of  the 
two  forces — i.e.  of  the  force  exerted  by  the  aerial  in 
one  direction,  and  the  force  exerted  by  the  stay  in 
another  direction — is  a  force  acting  straight  down  the 
length  of  the  mast. 


FIG.  116. 


192 


WIRELESS  TELEGRAPHY 


670.  A  very  simple  way  of  calculating  the  amount  of 
the  force  acting  on  the  stay  and  that  acting  on  the  mast 
is  by  drawing  a  parallelogram,  as  shown  in  Fig.  118: 

Assuming  that  the  aerial  is  exerting  a  horizontal 
pull  of  200  lb.,  and  we  take  1  inch  to  represent  a  pull 


PULL  or 


Fio.  117. 


of  100  lb.,  then  we  may  set  out  a  horizontal  line,  AB. 
2  inches  long,  to  represent  the  pull  of  the  aerial. 

671.  The  force  exerted  by  the  mast  will  be  in  a 
vertical  direction  ;  therefore  we  may  set  out  a  vertical 
line,  AC,  of  indefinite  length,  representing  the  direction  of 
the  force  exerted  by  the  mast.  Further,  we  may  draw 
a  line,  AD,  representing  the  direction  of  the  force  exerted 
by  the  stay,  the  angle  oc  the  same  as  the  angle  formed 
by  the  stay  and  the  mast. 

If  now  we  draw  from  the  point  B  a  line  parallel  with 
the  line  AD,  this  line  will  cut  the  line  AC  at  the  point 
E,  and  the  length  of  the  line  EA  in  inches  will  represent 
the  force  exerted  on  the  mast  in  hundreds  of  pounds. 


WIRELESS  TELEGRAPHY 


193 


Further,  if  we  draw  from  the  point  E  a  line  parallel 
with  the  line  BA,  this  line  will  cut  the  line  AD  at 
the  point  F,  and  the  length  of  the  line  AF  in  inches 
will  likewise  represent  the  force  in  hundreds  of  pounds 
exerted  by  the  stay. 

672.  Measuring   these   lines  in  the   particular  case 


Fio.  119. 

shown  in  Fig.  118,  where  the  angle  between  the  mast 
and  the  stays  is  30°,  we  find  that  the  line  AE  is  about 
3|  inches  long,  and  therefore  the  force  required  to  be 
exerted  by  the  mast,  or  the  pressure  on  the  mast,  is 
350  lb.,  while  the  line  AF  is  about  4  inches  long,  and 
therefore  the  force  required  to  be  exerted  by  the  stay, 
or  the  pull  on  the  stay,  is  400  lb. 

673.  By  making  similar  diagrams  for  various  angles 
between  the  mast  and  the  stay,  as  shown  in  Fig.  119,  it 
will  be  seen  that  the  greater  the  angle  the  less  the  strain 
both  on  the  mast  and  on  the  stay  for  a  given  aerial 
pull. 


194  WIRELESS  TELEGRAPHY 

674.  Obviously,  then,  it  is  an  advantage  to  increase  the 
angle  at  which  we  stay  a  mast,  more  especially  in  the 
case  of  portable  masts,  where  anchor  pegs  have  to  be 
used  for  attaching  the  stays  to  the  ground,  and  in  soft 
ground  a  comparatively  small  pull  would  be  required 
to  pull  them  out  of  the  ground. 

There  are,  of  course,  practical  limitations  to  the 
extent  to  which  we  can  do  this,  for  if  >ve  make  the 
angle  too  great,  not  only  is  a  large  open  space  required 
in  which  to  erect  the  mast,  but  also  the  necessary 
length  of  the  stays  increases  very  rapidly  after  an  angle 
of  about  30°  is  reached. 

675.  In  practice  it  is  usual  to  make  the  distance  from 
the  foot  of  the  mast  to  the  anchor  peg  equal  to  half  the 
length  of  the  mast.     The  angle  between  the  mast  and 
the  stay  is  then  about  27°. 


BUCKLING  OP  MASTS 

676.  Let  us  now  make  a  few  further  experiments  with 
jw  the  thin  stick  described  in  paragraph 

666.     If  we  take  two  such  sticks  of 
exactly    the    same    diameter    and 
length,  one  of   which   is  perfectly 
straight  and  the  other  very  slightly 
curved,  as  shown  in  Fig.    120,  it 
will   be   found    that    the    straight 
stick  will  carry  a  far  greater  weight 
than  the  bent  stick.     If  we  increase 
FIG  120          ~  *^e  wei§h*  on  the  bent  stick  gradu- 
ally, and  carefully  watch  the  effect, 
it  will  be  seen  that  the  bend  increases  gradually  as  the 
weight  is  increased,  until  it  reaches  a  certain  critical 


WIRELESS  TELEGRAPHY  195 

bend,  depending  upon  the  nature  of  the  wood  of  which 
the  stick  is  made.  As  soon  as  this  critical  point  is 
reached,  a  small  increase  in  the  weight  will  cause  the 
stick  to  collapse  and  break. 

We  will  suppose,  for  the  purpose  of  explanation,  that 
this  critical  point  is  reached  when  the  weight  applied  is 
20  lb.,  and  that  21  Ib.  is  necessary  to  break  the  stick. 

677.  If  now  we  apply  a  weight  of  21  lb.  to  the  straight 
stick,  it  will  be  found  that  it  carries  the  weight  without 
any  sign  of  breaking.     If,  however,  we  apply  a  side 
pressure  in  the  middle  sufficient  to  start  a  slight  bend, 
the  stick  will  immediately  collapse  in  exactly  the  same 
way  as  the  other  stick.     Only  a  very  slight  side  pressure 
is  required  to  start  the  bend  or  "  buckle."     In  the  case 
of  a  mast  in  the  open,  the  pressure  of  the  wind  will  be 
found  quite  sufficient  to  start  a  buckle  ;  but  in  any  case, 
all  masts  made  up  of  a  number  of  loose  sections  will 
have  a  slight  bend  in  them  to  start  with,  owing  to  the 
play  between  the  plugs  and  sockets. 

678.  If  now  we  take  the  same  sticks  and  cut  down 
their  length  by  one  half,  it  will  be  found  that  exactly 
the  same  effects  will  be  produced  by  applying  pressure 
to  the  ends,  except  that  it  will  now  take  four  times  the 
weight  to  reach  the  critical  point. 

679.  In  practice  it  is  found  that  tJie  weight  a  given 
stick  or  mast  will  carry  is  inversely  proportional  to  the 
square  of  its  length.     By  staying  the  middle  of  a  stick 
or  mast  in  such  a  way  that  the  point  of  attachment 
of  the  stays  to  the  mast  cannot  move  sideways,  as  shown 
in  Fig.  121,  we  have  in  effect  converted  the  stick  into 
two  sticks,  each  of  half  the  length,  one  on  top  of  the 
other.     Thus,  by  staying  a  stick  or  mast  in  the  middle^ 
we  quadruple  the  weight  or  pressure  it  will  carry 


196  WIRELESS  TELEGRAPHY 

MAST  STAYS 

680.  The   material  of    which  the  stays   are   made 
depends    entirely  upon    circumstances.     For    portable 
masts  the  stays  must  be  very  flexible,  as  they  have  to 
be  coiled  up  on  to  drums  when  the  mast  is  dismantled. 

681.  For  masts  up  to  30  feet  in  height,  rope  stays  are 
the  most  suitable.     For  masts  higher  than  30  feet,  how- 
ever, it  is  better  to  use  metal  stays,  because  rope  shrinks 


FIG.  121. 

badly  when  it  is  wet  and  stretches  again  when  dry,  the 
result  being  that  if  a  mast  has  been  erected  when  every-- 
thing  is  dry  a  shower  of-  rain  will  shrink  a  long  stay 
sufficiently  to  pull  an  anchor  peg  out  of  the  ground 
and  allow  the  mast  to  fall.  If,  on  the  other  hand,  the 
stays  are  adjusted  when  they  are  wet,  they  will  stretch 
as  they  get  dry  and  allow  the  mast  to  buckle  badly  and 
perhaps  break. 

682.  For  long  stays,  then,  metal  should  always  be 


WIRELESS  TELEGRAPHY 


197 


ased,  and  for  portable  masts  phosphor  bronze  is  toimcl 
to  be  the  best  metal  for  the  purpose,  although  some- 
what expensive,  although  it  has  not  the  same  tensile 
strength  as  steel,  and  will  not  corrode  or  rust  when 
exposed  to  the  atmosphere  Steel  can,  of  course,  be 
galvanised  to  stop  rusting,  but  this  reduces  its  strength 
very  considerably,  .more  especially  in  the  case  01  unely 


stranded  wires.  In  order  to  make  the  metal  stays 
flexible  they  are  made  up  of  many  strands  of  fine  wire 

683.  When  vnelal  slays  are  used  they  must  be  carefully 
insulated  from  the  earth,  otherwise  oscillatory  currents 
will  be  induced  in  them  on  account  of  their  proximity 
to  the  aerial,  and  they  would  thus  absorb  a  large  pro- 
portion of  the  transmitted  energy,  and  thereby  reduce 
the  range  of  the  station. 

The  insulation  of  stays  does  not,  however,  require- 


198 


WIRELESS  TELEGRAPHY 


to  be  of  a  very  high  order,  a  short  length  of  rope  being 
in  most  cases  quite  sufficient ;  for  even  when  wet  its 
resistance  will  be  sufficiently  high  to  stop  any  oscillatory 
currents  in  the  stays.  Although  in  this  case  there  will 
be  a  certain  amount  of  leakage  to  earth,  the  energy  thus 
absorbed  would  not  be  sufficient  to  affect  the  efficiency 
of  the  station  to  any  appreciable  extent. 

In  tall  masts  of  200  feet  upwards,  where  the  stays  are 
necessarily  long,  it  is  usual  to  divide  the  stays  into  two  or 
more  sections  with  rope  lanyards,  as  shown  in  Fig.  122. 
Special  porcelain  insulators  are  also  used  for  this  purpose. 
684.  In  the  case  of  portable  masts  which  rarely  exceed 
70  feet  or  100  feet  in  height 
this   division  of    stays   is 
quite  unnecessary,  and  an 
insulator  consisting   of    a 
piece  of  rope  between  the 
stay  and  the  anchor  peg 
is  quite  sufficient. 

Some  means  of  adjust- 
ing the  length  of  the  stays 
is  necessary,  and  it  is  usual 
to  make  this  piece  of  rope 
serve  the  two  purposes  of 
insulating  the  stay  and 
providing  a  means  of  ad- 
justing its  length,  as  shown 
in  Fig.  123. 

685.  In  the  case  of 
wooden  masts  no  insulation  is  necessary  at  the  upper  end 
of  the  stay,  but  in  the  case  of  steel  masts  the  insulation 
of  the  stay  from  the  mast  is  even  more  important  than 
(he  insulation  of  the  stay  from  the  ground.  The  reason 


ROPE 


Fio.  123. 


WIRELESS  TELEGRAPHY 


199 


FIG.  124 


for  this  is  obvious  by  glancing  at  Fig.  124,  when  it  will 
be  seen  that  unless  „ 

the  stays  are  insu- 
lated at  the  points 
marked  "  A,"  the 
stays-,  together  with 
the  mast,  form  a 
fair-sized  umbrella 
aerial,  connected  to 
earth  through  the 
mast;  the  stays 
forming  the  radial 
wires  of  the  aerial 
and  the  mast  form- 
ing the  down-lead.  This  would  absorb  a  very  large 
amount  of  the  energy  radiated  from  the  aerial  proper. 
686.  In  the  case  of  steel  masts,  therefore,  it  is 

PLPTE 

POPE  ItHSULRTOf? 

&'     LOMi 

Wif?E 


FIG.  125. 

necessary  to  insert  insulators  between  the  mast  and  the 
stay.  For  portable  masts  a  length  of  about  6  inches  of 
rope  serves  the  purpose  very  well,  as  shown  in  Fig.  125. 

THE  INSULATION  OF  AERIALS 

687.  The  insulation  of  the  aerial  is  a  matter  of  the 
utmost  importance. 


200  WIRELESS  TELEGRAPHY 

Bad  insulation  means  a  leakage  of  current  and 
therefore  a  loss  of  power.  That  is  to  say,  instead  of 
radiating  all  of  the  energy  in  the  form  of  ether  waves, 
part  of  the  energy  will  be  lost  in  leakage  during  each 
oscillation.  The  effect  on  the  oscillating  current  is  to 
increase  the  "  damping,"  so  that  in  addition  to  loss  of 
power  we  get  flatter  tuning,  due  to  the  more  highly 
damped  waves  produced. 

688.  It  is  obvious  that  the  longer  the  energy  remains 
in  a  leaky  aerial  the  greater  will  be  the  loss  due  to  leakage. 
It  is  also  obvious  that  an  aerial  which  is  a  good  radiator 
will  not  retain  the  energy  put  into  it  for  so  long  as  an 
aerial  which  is  a  slow  radiator.     It  follows,  therefore, 
that  the  slower  an  aerial  radiates  its  energy  the  greater 
the  loss  of  energy  due  to  bad  insulation. 

689.  In  addition  to  the  rate  at  which  an  aerial  radiates, 
there  is  another  point  to  be  considered,  namely,  the  rate 
at  which  the  energy  is  put  into  the  aerial.     Assuming 
that  the  faulty  insulator  acts  as  a  conductor  with  a 
high  resistance  connected  to  earth,  then  the  rate  of 
leakage  from  the   aerial  will  be   proportional  to   the 
voltage  of  the  aerial  at  the  point  of  leakage,  for  the 
higher  the  voltage  the  greater  will  be  the  current  passing 
through  the  resistance  of  the  faulty  insulator. 

690.  Taking  the  case  of  a  transmitter  in  which  a 
primary  circuit  is  loosely  coupled  to  the  aerial  circuit,  the 
energy  in  the  primary  circuit  is  only  slowly  transferred  to 
the  aerial,  so  that  the  aerial  will  not  attain  its  maximum 
voltage  until,  perhaps,  the  third  or  fourth  oscillation. 
But  since,  during  these  oscillations,  the  aerial  is  radiating 
its  energy,  it  follows  that  the  maximum  voltage  it  will 
attain  will  not  be  so  high  as  if  the  two  circuits  were  so 
closely  coupled  together  that  the  whole  of  the  energy 


WIRELESS  TELEGRAPHY  201 

were  transferred  to  the  aerial  during  the  first  oscillation. 
Consequently  the  loss  of  energy  due  to  leakage  will  be 
less  in  the  case  of  the  loosely  coupled  circuits  than  in 
the  case  of  the  closely  coupled  circuits. 

691.  Although  faulty  insulation  is  bad  even  in  the 
case  of  loosely  coupled  circuits,  inasmuch  as  it  results  in 
less  power  being  radiated,  it  will  not  put  the  station 
completely  out  of  action,  for  oscillatory  currents  will  still 
be  induced  in  the  aerial,  and  therefore  waves  will  be 
radiated,  though  the  range  of  communication  may  be 
very  much  reduced.  This  is  one  of  the  reasons  that  give 
coupled  transmitters  such  a  great  advantage  over 
"  plain  aerial."  In  the  case  of  a  plain-aerial  transmitter, 
as  the  rise  in  voltage  across  the  secondary  of  an  in- 
duction coil,  when  the  primary  circuit  of  the  coil  is 
interrupted,  takes  an  appreciable  length  of  time,  the 
charge  in  the  aerial  may  leak  away  through  the  faulty 
insulators  as  fast  as  it  is  supplied  by  the  induction  coil, 
with  the  result  that  it  is  impossible  to  get  a  spark  across 
the  electrodes  from  aerial  to  earth.  And,  since  the 
current  in  the  aerial  is  not  oscillatory,  until  the  spark 
takes  place,  no  oscillating  currents  are  produced,  and 
therefore  the  aerial,  under  these  conditions,  does  not 
radiate  at  all. 

AERIAL  INSULATORS 

'692.  The  first  point  to  consider  in  connection  witt 
aerial  insulators  is  the  dielectric  strength  of  the  material 
of  which  the  insulator  is  made. 

693.  When  an  electric  pressure  is  applied  to  an  in- 
sulating material  or  dielectric,  a  mechanical  stress  is  set 
up  in  the  dielectric.  Further,  if  the  electric  pressure  is 
increased  beyond  a  certain  limit  (depending  upon  the 


WIRELESS  TELEGRAPHY 


thickness  and  nature  of  the  material),  the  dielectric  is 
broken  or  punctured  at  its  weakest  point.  If  the  di- 
electric be  a  liquid  or  a  gas,  the  puncture  is  only  moment- 
ary and  heals  up  automatically  as  soon  as  the  current 
ceases  to  flow  through  the  path  thus  made,  but  in  the 
case  of  solids  the  puncture  remains,  and  the  insulation  is 
permanently  broken  down  at  this  point.  The  voltage 
at  which  the  puncture  takes  place  for  a  given  thickness  of 
material  is  called  the  dielectric  strength  of  the  insulating 
material,  and  varies  considerably  with  different  materials. 
694.  The  following  table  shows'  the  comparative 
values  of  different  substances  in  this  respect. 


Substance. 

Voltage  required  to  puncture 
1  centimetre  thickness  of 
material. 

Air       .         .         .         .         .': 

30,000 

Oil       .         .         .         :         . 

60,000  to  80,000 

Ebonite  (best  quality)    . 

600,000 

Soft  india-rubber 

450,000 

Mica    .          .                    . 

1,000,000 

Glass   .                            . 

250,000 

Paraffin  wax 

170,000 

Porcelain      .                   . 

100,000 

The  above  figures  are  only  approximate  and  vary 
considerably  with  different  samples  of  the  same  material. 
In  any  case,  in  practice  it  is  advisable  to  allow  a  factor  of 
safety  of  at  least  3,  usually  more. 

695.  The  second  point  to  consider  in  connection  with 
aerial  .insulators  is  the  surface  insulation.  Even  when 
an  insulator  is  perfectly  dry,  at  high  voltages  electricity 
will  creep  over  the  surface  far  more  readily  than  it  will 
spark  across  an  air-gap.  Thus,  if  a  pressure  of  30,000 
volts  be  applied  across  two  metal  discs  separated  by 


WIRELESS  TELEGRAPHY 


203 


15  cms  of  an-,  as  shown  in  Fig  126,  no  discharge  will 
take  place  between  them,  but  if  the  same  space  be  filled 
with  ebonite  as  shown  in  Fig  127,  although  the  ebonite 
will  not  be  punctured  (see  table  of  dielectric  strengths 
in  paragraph  694),  the  electricity  will  run  along  the 
surface  of  the  ebonite  between  the  two  electrodes 


FIG    126 

696  In  order  to  increase  the  length  of  the  path  along 
the  surface  of  the  insulator  without  increasing  the  overall 
length  of  the  insulator,  it  is  usual  to  make  the  surface 


Fie;    127 

corrugated,  as  shown  in  Figs.  128  and  129,  thereby 
doubling  or  trebling  the  length  of  the  surface. 

In  the  case  of  aerial  insulators,  however,  the  overall 
length  is  not  a  matter  of  great  importance,  so  that 
corrugated  insulators  are  not  often  used,  the  insulators 
being  made  sufficiently  long  in  themselves. 

697  The  actual  length  of  surface  insulation  to  allow  is 
a  difficult  matter  to  determine,  as  everything  depends 
upon  the  nature  and  condition  of  the  surface.  Where 


204 


WIRELESS  TELEGRAPHY 


a  dry,  clean  surface  is  assured  at  all  times,  it  will  be 
quite  safe  to  allow  4  cms  of  surface  to  every  30,000  volts 
of  potential.  But  aerial  insulators  are  exposed  to  all 
kinds  of  weather  conditions,  and  if  the  surface  of  an 
insulator  be  allowed  to  get  coated  .with  a  film  of  moisture 


and  dirt,  almost  any  length  will  be  useless  for  the  purpose 
of  insulation. 

698.  In  most  cases  dirt  accumulates  slowly,  and 
trouble  from  this  source  can  be  avoided  by  a  periodic 
inspection  and  cleaning  of  the  insulators  The  chief  diffi- 
culty, therefore,  is  how  to  keep  the  surface  of  the  insulator 
dry.  When  the  insulator  occupies  a  more  or  less  vertical 
position  this  is  easily  accomplished  by  fitting  a  cone 
over  the  insulator  to  act  as  a  water-shed,  as  shown  in 


WIRELESS  TELEGRAPHY 


205 


Fig.  130 ;  but  when  the  insulator  occupies  a  horizontal 
position,  as  it  might,  for  example,  when  supporting  a 
horizontal  aerial,  this  method  would  obviously 
be  useless.  In  such  cases  it  is  usual  to  make 
the  insulator  of  ample  length  and  to  paint  its 
surface  with  a  bitumastic  varnish,  so  that 
any  moisture  settling  on  it  will  detach  itself 
into  separate  drops  instead  of  forming  a  con- 
tinuous film  of  moisture  over  the  whole  of  the 
surface. 

699.  The  surface  of  a  porcelain  insulator 
has  this  property  without   being  varnished, 
but  this  material  is,  unfortunately,  extremely 
brittle  and  therefore  unsuitable,  at  all  events 
for  portable  stations. 

700.  An  important  point  to  bear  in  mind  when  insulat- 


FIG.  130. 


FIG.  131. 


ing  an  aerial  is  to  use  as  few  insulators  in  parallel  as 
possible,  for  each  insulator  thus  used  increases  the  total 


206 


WIRELESS  TELEGRAPHY 


leakage  from  the  aerial.     Thus  an  aerial  insulated  as 
illustrated  in  Fig.  131  will  only  have  half  the  leakage  as 


FIG.  132 

insulated  as  shown  in  Fig.  132.  Moreover,  only 
half  the  number  of  insulators  are  required,  and  therefore 
it  is  less  costly. 

EARTHS 

701.  A  point  of  the  utmost  importance  to  the 
efficiency  of  communication  is  a  good  "  earth  "  at  both 
the  transmitting  and  receiving  stations. 

"  Earths  "  fall  under  one  of  two  headings,  namely,  (1) 
Direct  Earths  ;  (2)  Capacity  Earths  or  "  counterpoise." 

We  have  already  shown,  in  paragraph  651,  that  a 
node  of  current  exists  at  the  free  end  of  the  aerial,  and 
that  a  maximum  of  current  flows  at  the  earthed  end 
of  the  aerial.  This  current  must  flow  through  the 
earth  connection  from  the  aerial  to  the  earth,  and  from 


WIRELESS  TELEGRAPHY  207 

the  earth  to  the  aerial.  Therefore,  any  resistance  in  the 
earth  connection  will  cause  loss  of  power  and  damping 
of  the  oscillations. 

702.  In  the  case  of  ship  stations  the  "  earth  "  is  a 
simple  matter,  as  salt  water  is  an  excellent  conductor  of 
electricity,  and  it  is  therefore  only  necessary  to  attach 
a  conducting  wire  to  the  metal  hull  of  the  ship.    In  the 
case  of  land  stations,  it  is  usual  to  bury  a  number  of 
plates  of  zinc,  or  other  non-corrosive  metal,  at  a  sufficient 
depth  to  ensure  their  being  surrounded  by  damp  earth. 

703.  It  is  found  better  to  use  a  number  of  long  strips 
spreading  radially  under  the  soil  than  to  use  one  large 
plate,  as  the  former  gives  less  resistance  than  the  latter. 
The  reason  for  this  is  that  the  effective  resistance  of  a 
buried  "  earth  "  is  reduced  in  proportion  to  the  capacity 
of  the  earth  plates.     Thus,  the  greater  the  capacity  of 
the  earth  plates  as  a  whole  the  less  the  effective  resistance 
of  the  "  earth."     The  most  efficient  earth,  where  the 
resistance  of  the  soil  is  high,  is  made  by  burying 'a  large 
number  of  wires  below  the  aerial  or  burying  them 
radially  in  all  directions,  like  an  underground  umbrella 
aerial. 

704.  It  is  not  necessary,  however,  to  have  a  direct 
electrical  connection  with  the  earth:     What  are  known 
as  "  counterpoise  "  or  "  capacity  earths  "  are  frequently 
used    even    with    comparatively    high-power    stations. 
Such  an  "  earth  "  usually  consists  of  a  large  number  of 
wires  suspended  above  the  surface  of  the  ground  and 
carefully  insulated  from  it.     It  is  important  in  this 
case  that  the  capacity  of  the  counterpoise  should  be  at 
least  equal  to  that  of  the  aerial. 

705.  For  portable  stations  this  form  of  "  earth  "  is 
frequently  employed,  as  it  takes  less  time  to  erect  than 


208  WIRELESS  TELEGRAPHY 

it  does  to  dig  deep  trenches.  For  such  stations,  perhaps 
the  most  convenient  form  of  earth  is  a  number  of  long 
narrow  strips  of  wire-netting,  which  can  be  laid  on  the 
ground  star  shaped,  and  can  be  rolled  up  into  convenient 
rolls  for  transport  purposes.  Under  some  conditions, 
when  the  surface  of  the  ground  is  wet  and  conducting, 
the  "  earth  "  acts  as  a  direct  connection  to  earth,  while 
under  other  conditions,  when  the  surface  of  the  ground 
is  dry,  the  "  earth  "  acts  as  a  counterpoise. 

706.  This  form  of  earth,  although  not  as  efficient 
under  some  conditions  as  a  true  counterpoise,  has  the 
great  advantage  of  simplicity  and  ease  of  erection,  and 
moreover  does  not  interfere  with  the  approach  to  the 
station.  It  will  be  readily  understood  that  a  number 
of  wires  suspended  a  short  distance  from  the  earth  over  a 
wide  area  round  a  station  would  be  extremely  incon- 
venient in  a  military  camp,  where  a  stray  horse  might 
easily  become  entangled  with  it  at  night,  and  perhaps  do 
serious  damage  both  to  itself  and  the  station. 


INDEX 


Accumulators,  41 
Aerial  insulators,  201 
Aerials,   distribution   of  current 
in,  183-186 

distribution    of    potential  in, 
177-183 

effect  of  height  of,  172 

effective  capacity  of,  183*"* 

excitation  of,  84-96 

functions  of,  167 

fundamental   wave-length   of, 
170 

harmonics  in,  187 

indirect  excitation  of,  93 

insulation  of,  199 

length  of,  175 

shape  of,  168 

variation   of   wave-length   of, 

78-81 

Aether,  48 
Aether  waves,  49  f 

communication  by,  51 
Ampere,  17 
Atmospherics,  165 
Auto-jigger,  97 

Battery,  40 
Buzzer,  tuning,  146 

Capacities,  in  parallel,  effect  of,  82 

in  series,  effect  of,  79 
Capacity,  definition  of.  9 

inductive,  8 

inductive,  mechanical  analogy 
of,  8 


Capacity,  unit  of,  21 
Carborundum,     rectifying     pro- 
perties of,  115,  138 
Cell,  electrolytic,  155 
Circuits,  electrical,  14 
oscillatory,  closed,  75 
oscillatory,  coupled,  89 
oscillatory,  coupled,  mechani- 
cal analogy  of,  95 
oscillatory,  coupled,  reaction  of 

secondary  on  primary,  98 
oscillatory,  coupled,  resultant 

wave-lengths  of,  99 
oscillatory,  effect  of  resistance 

in,  69 

oscillatory,  energy,  70-75 
oscillatory,   essential  qualities 

of,  68 
oscillatory,  mechanical  analogy 

of,  68 

oscillatory,  open,  76 
oscillatory,  power  in,  75 
oscillatory,  variation  of  wave- 
length of,  77-83 
oscillatory,     wave-length     of, 

69 

receiver,  proportions  of,  127 
Coil,     induction,     characteristic 
oscillatory,  voltage  curve  of, 
39 

induction,  explanation  of,  34-40 
primary,  definition  of,  33 
secondary,  definition  of,  33 
Condenser,  adjustable,  construe- 
tion  of,  116 


209 


21C 


WIRELESS  TELEGRAPHY 


Condenser,  energy  stored  up  in,  75 

mechanical  analogy  of,  9 
Conductors,  5 
Contents,  vii,  viii 
Coulomb,  17 
Coupling,  calculation  of,  109 

effect  of  inductance  on,  108 

explanation  of,  106 

methods  of  varying,  110 
Crystals,  characteristic  curve  of, 
131 

rectifying  properties  of,  138 

use  of,  115 
Current,  electrical  unit  of,  17 

oscillatory,  detection  of,  120 

Damped  waves,  definition  of,  60 
Damping,  definition  of,  67 
Detector,  application  of,  in  re- 
ceivers, 120-121 

crystal,  use  of  potentiometer 
for,  122 

electrolytic,  151 

electrolytic,  circuits  of,  157 

in  wavemeter,  114 

magnetic,  159 

Earths,  206 

Electric  waves,  length  of,  64 

production  of,  61^-64 
Electricity,  mechanical  analogy 
of,  14 

production    of,    by    chemical 
action,  40 

production  of,  by  magnetism,29 
Electro-dynamics,  12 

definition  of,  3 
Electro-magnet,  28 
Electro-magnetic  field,  26 

induction,  26-29 
Electro -motive  force,  12 
Electro-static  field,  7 

induction,  6 
Electro-statics,  3 
Energy,  electrical  unit  of,  22 

in  oscillatory  circuits,  70-75 


Energy,  stored  up  in  a  condenser. 
75 

Farad,  definition  of,  21 
Field,  electro-static,  7 

magnetic,  25 
Force,  electro-motive,  12 
Frequency,   in   terms   of   wave 
length  and  velocity,  47 

of  waves,  46 

Harmonics  in  aerials,  187 
Height  waves,  production  of,  53 
Henry,  definition  of,  18 
High -resistance  telephones,  135 

Inductance,  mutual,  effect  of  in 
coupled  circuits,  104 

unit  of,  19 
Induction  coil,  34-40 

electro -magnetic,  29 

mutual,  32,  100 

static,  6 

Inductive  capacity,  8 
Insulation  of  mast  stays,  197 
Insulation,  surface,  203 
Insulators,  6 

aerial,  201 

corrugation  of,  201 

Jigger,  96 

auto-,  97 
Joule,  22 

Length  of  wave,  definition  of,  46 
in    terms    of    frequency    and 

velocity,  47 
Lines  of  Force,  direction  of,  25-26 

Magnetic  Detector,  159 

Field,  25 
Magnetism,  24 

production  of,  by  electricity,  27 
Magneto-motive  force,  28 
Mast  stays,  insulation  of,  197-199 

strain  on,  192 


WIRELESS  TELEGRAPHY 


211 


Masts,  buckling  of,  194 

staying  of,  196 

strain  on,  190 
Medium,  definition  of,  43 
Microfarad,  definition  of,  21 
Microhenry,  definition  of.  20 
Morse  Code,  ix 
Multiple  Tuner,  164 
Mutual  Induction,  32 

Inductance,  effect  of,  in  coupled 
circuits,  104 

Ohm,  18 

Ohm's  Law.  23 

Oscillatory  circuits,  closed.  7f> 

coupled,  89 

coupled,    mechanical    analogy 
of,  95 

coupled,  reaction  of  secondary 
on  primary,  98 

effect  of  resistance  m,  69 

energy  in.  70-75 

essential  qualities  of.  68 

mechanical  analogy  of,  68 

open,  76 

power  in.  75 

resultant     wave    lengths     of, 
99 

variation    of    wave-length    of. 
77-83 

wave-lengths  of,  69 
Oscillatory  transformer.  96 

Permeability.  28 
Potentiometer,    application     of, 
124 

explanation  of,  122 
Power,  unit  of,  23 
Pressure,  electrical  unit  of.  17 
Pressure  waves,  48 

mechanical  analogy  of,  56 

production  of,  54-64 
Properties  of  waves,  43 

Receivers,  118-164 
circuits,  proportions  of,  127 


Receivers,  detectors  of,  120 

essentials  of.  119 

single  circuit.  121 

telephone,  133 

telephone,  sound  produced  in, 
45 

two-circuit,  125 

tuning  of,  144 
Resistance,  unit  of.  '  ?• 
Reluctance.  28 

Spark-gap,  function  of,  87 

Static  induction,  6 

Stays,  mast,  insulation  of.  197 

mast,  strain  on,  192 
Symbols,  xi,  xn 

Telephones,  receiver.  134 
receiver,  high -resistance,  135 
receiver,   sound   produced   in, 

143 
Transmitters,     coupled     circuit, 

93 

plain  aerial,  89 
Tuning   Buzzer,   description   of, 

146 
use  of.  145. 

Units,  electrical,  16-23 

Velocity  of  wave.  46 
Volt.  17 

Watt,  23 

Wave,  amplitude  of,  46 
frequency  of.  46 
Idngth  of.  46 
velocity  of.  46 
Wave-lengths,     fundamental   of 

aerial,  169-172 

relation   to  capacity   and   in- 
ductance, 69 
use  of,  in  Wireless  Telegraphy, 

64 

variation    of,    in    aerial,    77 
81 


212 


WIRELESS  TELEGRAPE[Y 


Wave-lengths,   variation    of,   In 
closed    oscillatory    circuits, 
82-84 
Wavemeter,  description  of,  111- 

114 
Wave-motion,  43 

communication  by,  44 
Waves,  aether,  effects  produced 

by,  49 

continuous,  definition  of,  00 
damped,  definition  of,  60 


Waves,  electric,  length  of,  64 
electric,  production  of,  61-64 
height,  production  of,  53 
measurements  of,  45 
pressure,  48 
pressure,  laws  of,  48 
pressure,  mechanical  analogy 

of,  56 
pressure,    production   of,    64- 

66 
properties  of,  43 


THE    END 


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